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Cytotoxic T cells specific for Sendai virus were generated by culturing murine spleen cells in vitro together with UV-inactivated Sendai virus. In vivo immunization of donor mice with UV-inactivated Sendai virus resulted in an in vitro secondary response of increased magnitude. Cytotoxic activity was demonstrated in a short-term 51Cr-release assay, using syngeneic tumor cells which had been coated with inactivated Sendai virus by incubation at 4 degrees C for 30 min. The lysis of Sendai virus-coated target cells was restricted by the H-2 haplotype of the target cells, suggesting that the H-2 genes of the target cell contributed to the specificity of the lysis. Kinetic experiments showed that susceptibility to lysis by cytotoxic T cells specific for Sendai virus appeared within 30 min after coating target cells with inactivated virus. Furthermore, there was no detectable synthesis of new proteins in cells treated with UV-inactivated Sendai virus. For these reasons, we suggest that neither viral replication nor the synthesis of new proteins are necessary for the production of the antigen recognized by cytotoxic cells specific for Sendai virus. We infer that the virus-specific component on the target cells is probably a preformed virion antigen adsorbed onto or integrated into the cell membrane. These results imply that, if the cytotoxic T cell recognizes a single antigenic determinant specified both by viral and H-2 genes, this determinant is formed by the physical association of H-2 and Sendai virus antigens rather than by their alteration during the processes of synthesis.

To determine the extent and nature of individual variation in the development of specific antigen-binding cells, the numbers of cells specific for each of two antigens in the spleens of individual random-bred Swiss-L and inbred CBA/J and BALB/c fetal mice were measured as a function of spleen size. For Swiss-L fetuses, the ratio of antigen-binding cells to nucleateated cells varied more than would arise from sampling fluctuation. For each inbred strain, however, the number of cells specific for a given antigen was a constant proportion of the total number of nucleated cells within sampling error. These proportions varied from antigen to antigen, and from strain to strain. The ratio of the proportions of cells specific for the two antigens, however, differed no more from CBA/J to BALB/c mice than would be expected in repeated samples of cells from the spleen of a single fetus. These results confirm at the level of the individual fetus the uniform pattern of development seen for populations of fetuses. They reveal a surprising precision in the proliferation of specific antigen-binding cell populations and suggest that the development of these cells may be subject to strong genetic controls.

Cytotoxic T lymphocytes were generated in vitro against H-2 compatible or syngeneic tumor cells. In vitro cytotoxic activity was inhibited by specific anti-H2 sera, suggesting that H-2 antigens are involved in cell lysis. Two observations directly demonstrated the participation of the H-2 antigens on the tumor cells in their lysis by H-2-compatible T cells. First, coating of the H-2 antigens on the target tumor cell reduced the number of cells lysed on subsequent exposure to cytotoxic T cells. Second, when cytotoxic T cells were activated against an H-2 compatible tumor and assayed against an H-2-incompatible tumor, anti-H-2 serum that could bind to the target cell, but not to the cytotoxic lymphocyte, inhibited lysis. H-2 antigens were also shown to be present on the cytotoxic lymphocytes. Specific antisera reacting with these H-2 antigens, but not those of the target cell, failed to inhibit lysis when small numbers of effector cells were assayed against H-2-incompatible target cells or when effector cells of F1-hybrid origin and bearing two H-2 haplotypes were assayed against a tumor cell of one of the parental strains. These findings suggest that it is the H-2 antigens on the tumor cell and not those on the cytotoxic lymphocytes that are important in cell-mediated lysis of H-2-compatible tumor cells.

High doses of Concanavalin A (Con A), which normally inhibit T-lymphocyte stimulation as measured by increases in DNA synthesis, cause these lymphocytes to become committed to mitogenesis while also generating a dominant but reversible negative growth signal. The observed response to the stimulatory signal as measured by the rate of commitment to enter the S phase (i.e., the rate at which the stimulation becomes lectin independent) increases with lectin concentration even in the inhibitory range. The generation of this positive signal is prevented by treating the cells with colchicine. Cells that have become committed but are also simultaneously blocked from entering the S phase by the high doses of Con A can begin synthesizing DNA if the lectin is released by adding a competitive inhibitor of binding. Experiments done in agarose cultures in which lymphocytes are kept from contact with each other suggest that the reversible inhibitory signal is mediated by structures in the individual cells rather than as a result of agglutination. Continuously dividing cells of the lymphoid line P388 are also individually and reversibly inhibited by Con A. These findings are considered in terms of the relation of the inhibitory signal to the microtubular components of cell surface modulating assemblies made up of submembranous arrays of microtubules, microfilaments, and associated proteins.

Synthesis of H-2 antigens by preimplantation mouse embryos is first detectable at the late blastocyst stage. These antigens were detected using immune precipitation assays of extracts of embryos labeled by incorporation of radioactive amino acids but not by surface iodination. Experiments using isolated inner cell massess and trophoblast vesicles indicate that it is the cells of the inner cell mass that synthesize these antigens. H-2 antigens were not detected in either early blastocysts or at earlier cleavage stages.

A new method of affinity chromatography using glutaraldehyde-fixed cells immobilized on Sephadex beads has been used to isolate immunoglobulins (Ig's) specific for cell surface glycoproteins. Ig's that specifically bound and agglutinated the same cells as those originally fixed on the columns were isolated from nonimmune sera of various species. Periodate treatment of the cell-columns and the free cells destroyed their ability to bind the Ig's, and the binding of the Ig's to untreated cells was inhibited by monosaccharides such as D-galactose and sialic acid. The binding of antibodies directed against cell surfaces obtained by immunizing animals with the same mouse tumor cell lines used on the columns (P388 and EL4) was not inhibited by various saccharides. Surface glycoproteins obtained from the mouse tumor cells by immunoprecipitation with the column-isolated Ig's yielded specific electrophoretic patterns that differed from those obtained using Ig's from the sera of rabbits immunized with the tumor cells. The data suggest that the Ig's isolated by cell-column chromatography were directed against carbohydrates, probably those in terminal positions of the polysaccharide portions of the tumor cell surface glycoproteins. Column-isolated Ig's specific for carbohydrates were also useful in studies of cell interactions in nonmammalian systems including Dictyostelium discoideum and Saccharomyces cerevisiae. The cell-column method appears to be adaptable to the isolation of a variety of molecules in addition to antibodies.

Antibodies reactive with cell surface carbohydrates were isolated from normal chicken serum and were found to be mitogenic for mouse splenic lymphocytes as assayed by both blast transformation and [3H]thymidine incorporation. The Fab' fragments of these carbohydrate-binding immunoglobulins were just as mitogenic as the divalent native antibody. Moreover, succinylated Fab' fragments, which probably would not form self-associating aggregates, showed similar mitogenic properties. All of these results indicate that, at least for saccharide-specific ligands, multipoint attachment and receptor cross-linkage on the cell to which the ligand is attached may not be a stringent requirement for activation.

In order to analyze the development of antibody diversity in which the genes coding for the antigen-specific cells we have compared the binding of diverse antigens by cells in the fetal, neonatal, and adult mouse. Although the numbers of antigen-binding cells present in fetuses and young animals were smaller than in adults, no restriction could be detected in the varity of specificities expressed in the fetuses, either with respect to the kinds of antigens bound, or to the range of avidities of binding. Cells specific for each of the 11 antigens tested could be detected in the fetus only in the last 4 days before birth, at which time they appeared both in the liver and in the spleen. In all cases, these cells disappeared both in the liver and in the spleen. In all cases, these cells disappeared from the liver within a day of birth, but continued to increase in number in the spleen until adulthood...

Additional evidence has been obtained to show that different guinea pig anti-hapten antibodies differ in the structure of their L polypeptide chains. Antibodies from animals immunized with the same hapten conjugated to different carrier proteins gave similar starch gel electrophoretic patterns after dissociation of their chains. In a study of fine differences of specificity, cross-reacting antibodies were found to have some L chains with the same electrophoretic mobility. The multiplicity of L chain bands found in the characteristic starch gel electrophoretic patterns of dissociated anti-DNP antibodies was shown to be a reflection of the heterogeneity of antibodies of slightly different specificities. Reduction and alkylation of the active fragment produced by digestion of antibodies with papain yielded starch gel electrophoretic bands corresponding in mobility to L chains. The results are consistent with the notion that L chains are involved in the acquisition of immunologic specificity.

The antigenic properties of the polypeptide chains of human 7S gamma-globulin have been related to two major non-cross-reacting antigenic determinants of the whole molecule. These determinants, called S and F, were obtained by hydrolysis of gamma-globulin with papain. Antisera against whole gamma-globulin and against S and F fragments were used in techniques of immune diffusion. Light (L) chains of gamma-globulin showed reactions of partial identity with S fragments, and thus are antigenically deficient with respect to these fragments. Antisera directed against F determinants did not react with L chains but did react with heavy (H) polypeptide chain preparations. In addition, the major component of H chain fractions did not appear to contain determinants in common with the S fragments. L chains of a gamma-myeloma protein were shown to be antigenically deficient with respect to the whole myeloma molecule, and antigenically identical with the Bence-Jones protein of the same patient. Correlation of these results with those of previous investigations have led to the conclusions that the S fragment which is known to contain the combining region of antibody molecules, consists in part of L chains or portions of L chains, and that the F fragment, which mediates several other functions of the whole molecule, is composed in part of portions of H chains.

In order to clarify the cellular events that precede the onset of immunological competence in the mouse, we have characterized and quantitated the lymphoid cells of the spleen as a function of age. Our results show that T cells and B cells both appeared in the spleens of Swiss-L mice as early as the 15th-16th day of gestation. Antigen-binding cells specific for each of three different antigens were also first detected during this same 24 h interval. The B cells and three varieties of antigen-binding cells increased in number rapidly and in parallel until about 1 wk after birth. The T cells, which were more numerous than B cells at first, increased in number somewhat more slowly. Coincident with the onset of response to antigen, there was a further increase in B cell numbers and a decrease in the T cell to B cell ratio. The capacity to respond to antigen by cellular proliferation and synthesis of antibody did not arise until about 2 wk after birth although there were no quantitative changes in the total numbers of T cells, B cells, and antigen-binding cells between 1 and 2 wk of age. Some qualitative change, such as the functional maturation of an antigen-reactive cell, may be required during this interval for the onset of this immunological response. Although the numbers of antigen-binding cells present in fetuses and young animals were smaller than in adults, we have as yet been unable to detect any restriction in the variety of specificities that can be expressed in fetuses, either in the kinds of antigens bound or in the range of avidities with which a single antigen is bound.

The L polypeptide chains of certain Bence-Jones proteins of group I have been found in three forms: monomers of molecular weight of about 20,000, dimers which monomerize in dissociating solvents, and dimers which are stable in such solvents. The L polypeptide chains of some Bence-Jones proteins of group II were found to occur naturally only as stable dimers. The L chains of normal human gamma-globulin have been obtained in a reduced unalkylated form, and a fraction of these chains was found to form stable dimers under oxidizing conditions. It is suggested that a single disulfide bond is involved in stabilization of the dimer. In experiments on the reconstitution of 7S gamma-globulin, it was found that stable dimers of L polypeptide chains did not associate appreciably with H(gamma) chains to form a soluble product. L chains in the monomeric form, both of a reduced alkylated Bence-Jones protein and of reduced unalkylated gamma-globulin, combined with H(gamma) chains to form a 7S product. After hydrolysis with papain, the 7S material containing the Bence-Jones L chains yielded fragments comparable to the fragments of papain-treated myeloma proteins. As indicated by spectrofluorometric measurements, dissociable dimers and stable dimers of the L chains of a Bence-Jones protein both underwent identical thermally induced transitions in the temperature range 48-58 degrees C. When L polypeptide chains were present in reduced alkylated gamma-globulin or reduced alkylated S fragments, no transition occurred until 65 degrees C, the coagulation temperature of gamma-globulin and S fragments. Above this temperature, L chains were released into solution. These experiments suggested that free L chains and L chains bound to H(gamma) chains have different conformational stabilities.

The elasmobranch Mustelus canis has been shown to produce antibodies to Limulus hemocyanin. The serum of both normal and immunized M. canis contains immunoglobulins having sedimentation coefficients of approximately 7S and 17S. Antibody activity was found in the 17S immunoglobulin which may be dissociated to 7S components with concomitant loss of activity. Both 17S and 7S serum, immunoglobulins were antigenically identical. They consisted of light and heavy chains present in amounts comparable to those of higher vertebrates. Peptide maps indicated that the light chains had an entirely different primary structure than the heavy chains, but that the corresponding chains of 7S and 17S dogfish serum immunoglobulins were similar in primary structure. The heavy chains appeared to resemble the n chains of immunoglobulins of higher vertebrates in their starch gel electrophoretic behavior. It is suggested that the elasmobranch M. canis may have only one major class of immunoglobulins resembling that of macroglobulins (gammaM-immunoglobulins) seen in higher vertebrates. The results indicate that the multichain structure of antibodies is an ancient evolutionary development.

In spite of the prenatal appearance of immunoglobulin-bearing lymphocytes and theta-positive lymphocytes in the spleens of Swiss-L mice, these mice are not able to produce detectable levels of humoral antibodies in response to antigen until after 1 wk of age. Adult levels of response are not achieved until 4-8 wk of age. In the presence of bacterial lipopolysaccharides, which can substitute for or enhance T-cell function, the B cells from young Swiss-L mice were found to be indistinguishable in function from adult B cells, both with respect to the numbers of plaque-forming cells (PFC) produced in vitro in response to antigen and with respect to the kinetics of PFC induction. The spleen cells from young Swiss-L mice are significantly less sensitive than adult spleen cells, however, to stimulation by the T cell mitogens, concanavalin A (Con A) and phytohemagglutinin (PHA). Very few Con A-responsive cells could be detected at birth but the numbers increased sharply with age until 3 wk after birth. On the other hand, PHA-responsive cells could not be detected in the spleen until about 3 wk of age. The latter cells were found to respond also to Con A, but at a lower dose (1 microg/ml) than that required for the bulk of the Con A-responsive cells (3 microg/ml). The cells that respond both to PHA and to Con A appear in the spleen at about the time that Swiss-L mice acquire the ability to produce humoral antibodies, and these cells can be depleted from the spleen by the in vivo administration of antithymocyte serum. The development of humoral immune responses in these mice therefore appears to be correlated with the appearance of recirculating T lymphocytes that are responsive both to PHA and to Con A.

The relationships between the polypeptide chains of gammaG immunoglobulin and fragments of the molecule produced by papain and pepsin have been investigated. Specific procedures were employed including peptide mapping of tryptic hydrolysates and analysis of molecules reconstituted from chains labeled with different iodine isotopes. By these means, the Fab fragment was shown unequivocally to consist of the light chain and a portion of the heavy chain, the Fd fragment. The Fc fragment was found to be comprised of the residual portions of the heavy chain. These findings support the gross arrangement of chains embodied in recent models of the gammaG immunoglobulin molecule. The present studies have also provided additional information on the susceptibility of gammaG immunoglobulin to proteolytic cleavage. It was found that the portion of heavy chains corresponding to the Fd fragment was extensively cleaved by papain.

Specific precipitates and purified human antibodies were reduced and alkylated and subjected to starch gel electrophoresis in 8 M urea to dissociate and separate the L and H polypeptide chains. Dissociated antibodies to dextran, levan, teichoic acid, blood group A substance, and tetanus toxoid showed sharp bands corresponding to L polypeptide chains. The patterns differed for antibodies of different specificities. Some differences were also seen among antibodies of the same specificity from different individuals. Purified antidextran antibodies showed particularly simple patterns resembling those of purified human gamma myeloma proteins. In some cases only one L chain band was present.

The anuran amphibian, Rana catesbiana, has been found to possess at least two kinds of immunoglobulins corresponding to gammaG- and gammaM-classes. These classes have the same chain structures as those of their counterparts in higher animal species. Light chains of both immunoglobulins had molecular weights of 20,000. Heavy chains of the gammaM-class had molecular weights of 72,100; those of the gammaG-class had molecular weights of 53,600. The carbohydrate content of the gammaG-immunoglobulin was 2.1%, and that of the gammaM-protein was 10.8%. The amino acid compositions of the immunoglobulins were generally similar to those of mammalian immunoglobulins. After a single injection of phage antigen (f2), the order of appearance of phage-neutralizing activity in the frog immunoglobulin classes was (a) gammaM-antibodies, and (b) gammaG-antibodies. The results of this and previous studies suggest that the gammaG-immunoglobulins emerged at some point in evolution between the elasmobranchs and the anuran amphibians.

L polypeptide chains of myeloma globulin and Bence-Jones protein isolated from the same patient were found to be identical after comparison of their tryptic hydrolysates by two-dimensional high voltage electrophoresis. The patterns of peptides from proteins belonging to antigenic group I differed markedly from those of proteins in antigenic group II. A partially purified H chain fraction was compared with L chains from the same myeloma protein. The tryptic hydrolysates yielded dissimilar patterns of peptides. These data indicate that gamma-myeloma proteins contain two kinds of polypeptide chains, Hgamma chains and either L(I) or L(II) chains. The L chains appear to be identical with those comprising the Bence-Jones protein from the same patient.

When human and rabbit 7S gamma-globulins were reduced in strong urea solutions by a number of procedures, their molecular weights fell to approximately (1/3) of the original values. Partial separation of the reduction products was achieved using chromatography and starch gel electrophoresis in urea solutions. One of the components of reduced human 7S gamma-globulin was isolated by chromatography, identified by starch gel electrophoresis, and subjected to amino acid analyses. The amino acid composition of this component differed from that of the starting material and also from that of the remaining components. A reduced pathological macroglobulin dissociated to components with an average molecular weight of 41,000. Several reduced human myeloma proteins, when subjected to starch gel electrophoresis, yielded individual patterns that nevertheless had features in common with those of reduced normal gamma-globulins. Reduction of normal and abnormal gamma-globulins was accompanied by the appearance of titratable sulfhydryl groups. Chemical treatments other than reduction were used to determine the type of bond holding the subunits together. It was tentatively concluded that they were linked by disulfide bonds. An hypothesis is presented to relate the structural features of the various gamma-globulins in terms of the multiplicity of polypeptide chains in these molecules.

The effect of highly purified rheumatoid factor on the precipitin reactions of various antigen-antibody systems was determined. The amount of nitrogen precipitated was increased over a broad range when the factor was added to ovalbumin, human albumin, or human gamma globulin, and the corresponding rabbit antibodies. In the zone of antigen excess, soluble antigen-antibody complexes were precipitated by rheumatoid factor. Soluble aggregates of human and rabbit gamma globulin, produced by heating at 63 degrees C., treatment with urea plus mercaptoethanol or treatment with guanidine, also precipitated with rheumatoid factor. Ultracentrifugal analysis of dissolved specific precipitates showed the presence of aggregated gamma globulin. The sedimentation rate of reactive aggregates was greater than 20 S, and concentrated preparations free of the non-reactive 7 S gamma globulin could be prepared by various procedures of zone centrifugation. These aggregates showed a high inhibitory capacity in the sensitized sheep cell agglutination reaction. Solid gamma globulin, prepared by heat denaturation, also selectively adsorbed the rheumatoid factor, and removed or decreased the activity in the various precipitation and agglutination reactions. Elution of highly purified active preparations from the solid gamma globulin could be carried out with urea or acid buffers. Evidence for interaction between rheumatoid factor and low molecular weight gamma globulin without precipitation, was also obtained. This interaction appears to occur in the circulation of patients with rheumatoid arthritis. The question of whether the rheumatoid factor represents an antibody to gamma globulin was discussed. Points of similarity to the behavior of complement also were cited.

Two major antigenic fragments were obtained from various purified gamma-globulin preparations after papain treatment. One, the F component, had a mobility faster than the original gamma-globulin and the second, the S component, had a slower mobility. Similar F and S components were also obtained with certain homogeneous myeloma proteins which were closely related to gamma-globulin immunologically. Additional minor antigenic components were detected with certain antisera. The technique of immunoelectrophoresis was particularly useful for bringing out the different antigenic constituents obtained after papain treatment. The F and S components as well as a midfraction were isolated by chromatography on DEAE-cellulose. These were immunologically homogeneous and could be utilized to absorb F and S antibodies from various antisera. The relative amount of F and S antibodies varied in different antisera from individual rabbits immunized with whole gamma-globulin. Whole gamma-globulin was separated by zone electrophoresis into a fast migrating and a more slowly migrating fraction. Each of these gave rise to F and S components after splitting with papain. The F components of the two gamma-globulins were similar in mobility, while the S components showed marked mobility differences although antigenically they were very similar. The mobility differences of the parent gamma-globulin appeared to be primarily related to the differences in the S component. Certain antisera against pathological gamma-globulins were found to give double lines with a wide variety of gamma-globulin preparations in agar diffusion. These were shown to be related to the F and S antigenic determinants of gamma-glubulin. This relationship was evident by a number of procedures utilizing both Ouchterlony plate techniques and immunoelectrophoresis. The question of whether these findings indicate heterogeneity of gamma-globulin in relation to the F and S antigenic components, or whether different antigenic groups on one molecule can give rise to separate lines in certain instances, is discussed.

Reconstitution of phage-neutralizing activity was observed after admixture of separated L and H polypeptide chains of purified antibodies obtained from single sheep or guinea pigs and directed against f1 and f2 phages. Hybrid mixtures of complementary chains of antiphage antibodies and gamma-globulins of the same animal showed less activity than the homologous mixtures of chains from antibodies of the same specificity. 7S interspecies molecular hybrids could be formed between L or H chains of sheep gamma-globulins and the complementary chains of guinea pig gamma-globulins. In contrast to the results obtained within a single animal species, mixtures of L or H chains of antiphage antibodies from one species with the complementary chains of antibodies to the same phage from the other species showed slight or negligible potentiation of neutralizing activity.

The chemical relations among Bence-Jones proteins, myeloma proteins, and normal gamma-globulins have been investigated by a variety of means. Starch gel electrophoresis in 8 M urea of reduced alkylated Bence-Jones proteins yielded patterns of bands corresponding to those of the light (L) polypeptide chains of the dissociated myeloma protein from the same patient. One instance in which this correspondence was found was chosen for extensive study. Chromatography on carboxymethylcellulose in 6 M urea was employed to isolate the light (L) polypeptide chains and heavy (H) polypeptide chains of the completely reduced and alkylated myeloma protein. Isolation of similarly treated Bence-Jones protein from the same patient corroborated the correspondence to the L chains of the myeloma protein. Amino acid analyses indicated that the compositions of the Bence-Jones protein and the L chains of the myeloma protein were identical. Moreover, the thermosolubility properties and spectrofluorometric behavior of the isolated L chains and Bence-Jones protein were similar. Ultracentrifugal analyses of the L chains of normal human 7S gamma-globulin showed that their molecular weight in 6 M urea was 20,000. In aqueous solution their molecular weight was 41,000, suggesting that they exist as dimers under these conditions. The L chains of normal human gamma-globulin were found to have reversible thermosolubility properties similar to those of Bence-Jones proteins. The H chains of normal human gamma-globulin did not share these properties. Using spectrofluorometric methods, characteristic molecular transitions were found upon heating Bence-Jones proteins and L chains. These transitions were indicated by an increase in the intensity of fluorescence at well defined temperatures as well as by reversible shifts in the wavelength of maximal emission. The findings suggest that Bence-Jones proteins are composed of L chains of the type found in normal and pathological gamma-globu]ins.

The sea lamprey, Petromyzon marinus, has been found to produce specific antibodies after immunization with bacteriophage f2. Antibody activity is localized in 6.6S and 14S fractions of lamprey serum. The 6.6S antibodies were purified by a combination of zone electrophoresis, ion exchange chromatography, and gel filtration. Antigenic analysis of the 6.6S antibodies showed them to be free of other serum proteins and antigenically similar or identical to the 14S fraction. Evidence has been obtained which suggests that the 6.6S immunoglobulins consist of light components (molecular weight 25,000) and heavy components (molecular weight 70,000). In the immunoglobulin, these polypeptides appear to be linked via weak interactions but not by interchain disulfide bonds. Molecular weight analyses support the view that the chains can undergo concentration-dependent dissociation in aqueous solutions. Amino acid analyses showed that the compositions of the light and heavy components were similar and that aspartic acid or asparagine was the predominant amino terminal residue. Starch gel electrophoresis indicated that the subunits of lamprey antibodies are diffusely heterogeneous. The heavy chain mobility corresponded to that of micro-chains and resembled that of heavy chains of shark and sting ray immunoglobulins. In the course of the fractionation a 46S natural hemagglutinin composed of lower molecular weight subunits was isolated. This hemagglutinin did not resemble the lamprey immunoglobulin although it had a similar zone electrophoretic mobility in the beta-region. These studies are consistent with the hypothesis that micro-chains were the earliest of the heavy chain classes to emerge and further support the view that the multichain structure of immunoglobulins is a fundamental feature of antibody molecules.

Admixture of separated L and H polypeptide chains of 7S gamma-globulins under appropriate conditions has been found to result in the reconstitution of 7S molecules. The chains were mixed in 0.5 N propionic acid and when dialyzed into neutral aqueous buffers interacted to form reconstituted material in greater than 30 per cent yield. This material had sedimentation coefficients of 6S to 7S, a weight average molecular weight of 160,000, and its antigenic structure and electrophoretic properties were the same as those of 7S gamma-globulin. By the use of I(131) and I(125) labels on the different types of chains, combined with ultracentrifugation of chain mixtures in sucrose density gradients, the 7S product was found to contain both isotopes in ratios consistent with the presence of two L and two H chains. After hydrolysis with papain, the reconstituted material yielded products resembling S and F fragments. All of the isotope corresponding to L chains was found in the counterpart of the S fragment; the isotope corresponding to the H chain fraction was present in both fragments. The activity reconstituted from chains of a purified guinea pig antibody to f1 phage was found to be associated mainly with the 7S material. Hybrid molecules containing rabbit L chains and human H chains and of human L chains and rabbit H chains were formed by the same techniques of reconstitution. It was found that the interchain disulfide bonds of native 7S gamma-globulins could be broken and reoxidized, as could those of reconstituted 7S material. Reduced L and H chains mixed in propionic acid, dialyzed against neutral buffers containing mercaptan, then against neutral buffers in the absence of mercaptan, formed stable 7S molecules of molecular weight 160,000, which were dissociable only after reduction.