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The plaque-forming efficiency of Sindbis virus decreased as much as 1000-fold, and plaque size was diminished markedly, when tested on chick embryo cell monolayers aged in vitro. The plaquing efficiency and plaque size of Newcastle disease virus was unaffected. The reaction(s) associated with aging in vitro which lead to lowered plaquing efficiency are slowed considerably when cell monolayers are: (1) established in Simpson-Hirst medium rather than standard growth medium; (2) held at 31 degrees ; (3) given a regimen of daily medium changes; or (4) trypsinized and used as young secondary cultures. A loss in the average yield of virus per cell accompanies the loss in plaquing efficiency of Sindbis virus on aged monolayers. Adding actinomycin D to the aged cells at the time of infection eliminated completely the inhibition of Sindbis virus replication. Cells aged for 7 days in vitro were found to produce up to 32 times more interferon than cells 1-2 days old and were more sensitive to the action of interferon. The decrease in efficiency of Sindbis virus plaquing and yield in aged cells is accounted for by their development of an enhanced capacity to synthesize interferon upon appropriate stimulation. The process of contact inhibition and its concomitant regulation of macromolecular synthesis seems implicated in the aging phenomenon in that it may produce a generalized state of "enhanced derepressibility" in the cell.

Three aggregational forms of arylamidase are produced by Cephalosporium acremonium. The exocellular enzyme, with an approximate molecular weight of 60,000, was purified 300-fold by diethylaminoethyl cellulose chromatography, gel filtration, and gel electrophoresis. With l-leucyl-beta-naphthylamide as the substrate, the K(m) is 4.2 x 10(-4)m; the optimum pH, 7.7; and the temperature optimum, 35 C. The enzymatic hydrolysis of l-leucyl-beta-naphthylamide is inhibited by a number of cephalosporins, whereas a variety of penicillins show no effect. Alternatively, the enzyme specifically catalyzes the beta-lactam hydrolysis of a number of cephalosporins; a number of penicillins are resistant. The K(m) for cephalosporin C is 9.09 x 10(-4)m.

A hybrid mouse-hamster cell line was developed from a mouse cell line which produces a high titer of interferon and is sensitive to its action, and a hamster cell line which produces little interferon and is relatively insensitive to its action. Parental cell lines demonstrated complete species specificity with respect to interferon production and action. The hybrid cells produced interferon (or interferons) effective when tested on the mouse cell line and primary hamster cells; the hybrids were sensitive to the action of both mouse and hamster interferons. Hybrid cells produced ten times more hamster interferon than the parent hamster cell line and were eight times more sensitive to hamster interferon than the parent hamster cell line.

Cytomegalovirus (CMV) was demonstrated to induce intrinsic interference to Newcastle disease virus (NDV) in human fibroblast cells under noncytopathic conditions. This interference is unique in that (i) cytomegalovirus is the first DNA virus demonstrated to have this property and (ii) the state of interference was transient and progressively lost as the condition of the cells changed with the development of cytopathic effect. These observations are consistent with the view that the newly formed protein responsible for interference with NDV has a limited half-life and is no longer made when cytopathic conditions are produced by CMV.

The hemadsorption-negative plaque test has revealed a new type of viral interference, termed intrinsic interference. Several unrelated types of noncytopathic viruses were shown to induce in infected host cells a state of interference unique in being directed solely against superinfection by Newcastle disease virus (NDV). The NDV-refractory state arises only in those individual cells of a population actually infected by the inducing virus, and presumably results from the action of a protein(s) coded for by the viral genome. Thus, intrinsic interference differs fundamentally from that mediated by an extrinsic protein detectable under conditions favoring resistance to a broad spectrum of viruses and characteristic of interference induced by interferon, the latter being coded for by the cell genome. Intrinsic interference is defined as a viral genome-induced cellular state of resistance to challenge by high multiplicities of NDV, coexistent with a state of susceptibility to a broad spectrum of other viruses, similarly tested at high multiplicities. The capacity to induce intrinsic interference was demonstrated with rubella virus, Sindbis virus (arbovirus, group A), West Nile virus (arbovirus, group B), poliovirus (MEF, type 2), the lactic dehydrogenase virus (Riley's agent), and an unidentified nonhemadsorbing, noncytopathic adventitious virus. A state of intrinsic interference was also observed in the V5 line of mouse cells carrying a murine leukemia virus, probably resulting from some heretofore unsuspected contaminating virus. The molecular basis for intrinsic interference is not known, but it appears to involve a step in the NDV growth cycle beyond that of viral attachment, entry, and eclipse.

A simple and rapid plaque procedure has been developed for detecting and accurately assaying rubella virus in a noncytopathic virus-cell relationship. Plaque-formation is based on the development, in individual cells infected with rubella virus, of a unique type of intrinsic interference to infection with Newcastle disease virus. Rubella virus-infected cells challenged with Newcastle disease virus and tested for hemadsorption 15 hours later stand out as hemadsorption-negative areas. Individual living cells infected with rubella virus can be resolved under conditions allowing standard cloning procedures. In principle, the hemadsorption-negative plaque test can be used to search for a new class of noncytopathic, non-hemadsorbing viruses-those that induce an intrinsic interference to infection by any hemadsorbing virus.