Feline leukemia virus (FeLV) and simian sarcoma virus (SSV-1/SSAV-1) each contain six proteins which were separated by gel filtration in 6 m guanidine hydrochloride. The major protein both of FeLV and of SSV-1/SSAV-1 has species and interspecies antigenic determinants, and both viruses contain additional viral proteins with distinct species-specific antigenic determinant. The interspecies antigens of two primate C-type viruses (SSV-1/SSAV-1 and gibbon ape lymphoma virus) appear to be nearly identical and react only weakly with the corresponding antigens of lower animals.