Immunologically very closely related type C RNA viruses are endogenous to the domestic cat and to an old world primate, the baboon. In the present studies, radioimmunological techiques have been developed for detection of the 15,000 and 30,000 molecular weight (MW) polypeptides of each virus. The much more pronounced type specific antigenic determinants of the lower MW polypeptides made it possible to readily differentiate these viruses from each other as well as from a type C virus isolate from a second baboon species. Normal rhesus monkey tissues were partially purified and shown to contain a reactivity with MW and immunological properties similar to that of the baboon virus 30,000 MW polypeptide. Despite a similar degree of purification, antigenic reactivity like that of the baboon virus 15,000 MW polypeptide was undetectable even in the broadest immunological tests available for this polypeptide. The present findings indicate that the immunological properties of 2 structural polypeptides of closely related viruses endogenous to primate and feline species have undergone different rates of antigenic change in the course of evolution within their respective host cell genome.