Both lymphocytes and fibroblasts that have been transformed by ABelson murine leukemia virus contain 6- to 12-fold increased levels of the rare modified amino acid phosphotyrosine in their proteins. This observation, coupled with the fact that the p120 protein encoded by this virus has been shown to undergo an apparent autophosphorylation to yield phosphotyrosine in vitro, suggests that Abelson virus encodes a protein kinase that phosphorylates tyrosine in transformed cells. These results are similar to those obtained previously with Rous sarcoma virus and suggest, by analogy, that the modification of cellular polypeptides through the phosphorylation of tyrosine may be involved in cellular transformation by Abelson virus. p120 isolated from transformed cells contains phosphoserine, phosphothreonine, and phosphotyrosine. The phosphotyrosine is found at two sites in the protein. p120 therefore may be a protein kinase that undergoes autophosphorylation in vivo.