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The growth of hemopoietic cells of some inbred animal lines is repressed in hybrids of the first generation (F1)--the phenomenon of hybrid resistance. We have investigated the mechanism of hybrid resistance by studying the growth of hemopoietic cells of C57BL/6 mice in syngeneic and semisyngeneic heterotopic hemopoietic foci formed under the kidney capsule in (CBA*C57BL/6)F1 recipient mice. We found that hybrid resistance depended upon the joint involvement of NK cells and semisyngeneic hemopoietic stroma. It is concluded that NK cells recognize antigenic markers appearing on target cells located in a nonsyngeneic microenvironment.

A study was of the influence of size-fractionated thymocytes in the density gradient of bovine serum albumin on the number of CFUs in the bone marrow of mice exposed to sublethal irradiation. Subject to separation were thymocytes obtained from intact mice and mice after multiple injections of dexamethasone (DM). Two cell populations were demonstrated during separation of thymocytes from intact mice. Injection of the first cell population brought about a decrease in the number of CFUs in the bone marrow during postradiation recovery, whereas injection of other thymocyte subpopulation resulted in an increase in CFUs. The use of thymocytes obtained from DM treated mice enabled the detection of an increase in bone marrow CFUs, induced by both unfractionated thymocytes and two fractions of thymocytes.

Quantitative regularities have been revealed in the regulatory action of B-lymphocytes from mouse lymph nodes on killer activity of T-lymphocytes of low electrophoretic motility. It was shown that the radiation-induced changes in the mode of action of B-lymphocytes may be attributed to a decrease in the number of active cells.

The interaction of lymphocytes from mouse lymph nodes with allogeneic stem cells was studied using exogenous colony formation inhibition test. Dual nature of the interaction was revealed: great amounts of lymphocytes inhibited, while small amounts stimulated colony formation. This dependence holds true for macro- and microcolonies as well as for erythrocyte and granuloid microcolonies in the bone marrow during fixation on day 8 and 11 after cell mixture transplantation.

The preparative separation of lymph-node cells according to their charge and the study of the activity of each fraction have revealed that B-lymphocytes in the lymph nodes of mice have a regulating function in the phenomenon of the inactivation of allogeneic hematopoietic stem cells. The analysis of the dependence of effects, obtained in various experiments, on the ratio of B-lymphocytes and hematopoietic stem cells indicates that stem cells serve as targets for B-lymphocytes and the most probable mechanism of the above-mentioned effects is the direct interaction of the cells, one regulatory B-lymphocyte being capable of interacting with one target cell. Depending on the ratio of B-lymphocytes and stem cells, the inactivating effect may be either suppressed (the suppressor activity of B-lymphocytes) or enhanced (the helper activity of B-lymphocytes). The quantitative characteristics of the regulating activity of B-lymphocytes have been introduced.

A study was made of interaction of mouse spleen and lymph node lymphocytes in inactivation of allogeneic stem cells. It was established that T lymphocytes of the lymph nodes and spleen lymphocytes do not interact on combined administration; their action is of additive nature. B lymphocytes of the lymph nodes have a regulating activity both in respect to T lymphocytes of the lymph nodes and lymphocytes of the spleen. The stem cells serve as target. Depending on the stem cells/B lymphocytes ratio B lymphocytes are capable of exerting either helper or suppressor action.

A study was made of the changes in the mode of interaction between T- and B-lymphocytes of mouse lymph nodes with respect to the phenomenon of inactivation of non-syngeneic haemopoietic stem cells. It was shown that irradiation of B-lymphocytes with doses of 77.4--232.2 mC/kg changes their helper activity into a suppressor activity with regard to T-cell-killers having a low electrophoretic mobility.

In this work the B-cells of mouse lymph nodes are characterized. B-cells produce a helper effect on the capacity of the T-lymphocytes of the lymph nodes for inactivating nonsyngeneic stem cells. The study has revealed that the genetic heterogeneity of the B-lymphocytes does not lead to the abolition of their helper activity. B-lymphocytes of "B-mice" have also been shown to be capable of enhancing the inactivating activity of T-cells.