Antigenic markers associated with the major internal protein of RNA tumor viruses of the C-type have proven extremely useful in natural history studies of these viruses. This protein possesses species-specific antigenic determinants, and, in the case of mammalian C-type viruses, the protein possesses crossreactive determinants as well. These determinants are, thus, useful for species identification and classification of mammalian viruses. A unique distribution of antigens in embryonic tissues of several species (where tests are available) was detected, and in addition, antigen expression in tissues appears to be controlled by a dominant gene. These data have contributed greatly to the theory that RNA tumor-virus information is inherited as part of the cellular genome.