Highly sensitive and specific radioimmunosassays are described for quantitation of the intraspecies determinants of several mammalian C type viral group specific (gs) antigens. An interspecies (gs3) immunoassay has been developed which has both the broad reactivity and great sensitivity necessary for detection of C type viruses where intraspecies gs assays are not available. By using these immunoassays, the expression of endogenous virus specified gs antigens in mammalian cells of different species has been studied. Whereas mouse gs antigen was clearly detectable in tissue culture cells of several mouse strains, the respective gs antigens of rat, cat, Chinese hamster, woolly monkey, and gibbon ape were not detectable in cells of those species, using assays of comparable sensitivity. Thus, differences exist in the level of endogenous virus expression in cells of different mammalian species.