The ras oncogenes of the highly oncogenic Harvey and Kirsten murine sarcoma viruses encode the 21,000-dalton, p21, proteins. To elucidate the molecular mechanism of oncogenic activation, a metabolic turnover study of the p21 of EJ bladder carcinoma and its viral and normal cellular homologues has been carried out. It reveals the first functional difference between the p21 of the viral ras gene and its cellular homologues. Although these p21s appear to be synthesized by a very similar pathway and to have a similar subcellular distribution, the intracellular half-life of the viral p21 is much longer than that of the cellular p21 due to phosphorylation. The viral p21s of the highly oncogenic Ha-MuSV and Ki-MuSV differ from the cellular p21s by having specific point mutations at the phosphorylation sites, in addition to the mutations found at the 12th codon which activate the proto-oncogenes to the oncogenes found and occurring in many tumors. These studies indicate that there may be several steps in the activation of proto-oncogenes to oncogenes, of varying degrees of potency. Recently, the viral p21 oncogene product has been expressed at high levels in bacteria. As a result, several monoclonal antibodies against the p21 protein have been developed. The enzymatic activities of the bacterially produced p21 have been characterized using the purified protein. In other studies, partial revertants of Ha-MuSV-transformed MDCK cells have been isolated and characterized.