The only known product of the Snyder-Theilen strain of feline sarcoma virus (ST-FeSV) is a 85,000-dalton protein, designated ST P85, that contains feline leukemia virus gag gene encoded proteins (p15, p12, and a fragment of p30) and a sarcoma virus-specific polypeptide. Antibodies directed against the latter immunoprecipitated a 92,000-dalton phosphoprotein (NCP 92) expressed at low levels in normal feline embryo fibroblasts as well as in feline cells of epithelial or lymphoid origin. Normal cellular proteins crossreactive with ST P85 were also detected in cell lines from various other mammalian species. These results suggest that the ST-FeSV sequences encoding for the sarcoma virus-specific domain of ST P85 originated from an evolutionarily conserved cellular gene expressed in cells of independent differentiation lineage. Immunoprecipitates containing ST-FeSV P85 exhibited a protein kinase activity that specifically phosphorylated tyrosine residues. The physiological significance of this finding is illustrated by the finding that phosphotyrosine is an intrinsic component of ST P85. Furthermore, 5- to-fold higher levels of this unusual phosphorylated amino acid were present in ST-FeSV transformants than in uninfected control cells. Phosphorylation of tyrosine residues appears to be associated with cellular transformation caused by Rous sarcoma virus and Abelson murine leukemia virus. Thus, independent transforming virus isolates from birds, mice, and cats may utilize common pathways in exerting their oncogenic potential.