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The protein responsible for malignant transformation by avian sarcoma viruses (ASVs) has been identified as a phosphoprotein of molecular weight 60,000 designated pp60src (refs 1--4). It has been suggested that this protein has a functional role in cellular transformation involving the phosphorylation of cellular proteins, for it was discovered that specific immunoprecipitates from ASV-transformed cells that contain pp60src catalysed the transfer of phosphate from [gamma-32P]ATP to the heavy chain of rabbit immunoglobulin. Additional studies involving the cell-free synthesis of the ASV src protein further demonstrated that the presence of the src polypeptide correlated with that presence of a phosphotransferase activity. Our studies, involving the biochemical purification of this protein, have demonstrated that the ASV-transforming gene product, pp60src, is itself a protein kinase. We have purified the pp60src protein approximately 5,000-fold using either conventional ion-exchange chromatography or immunoaffinity chromatography. The resultant partially purified preparations contain a cyclic AMP-independent protein kinase activity. We report here that the soluble phosphotransferase activity of partially purified pp60src results in the phosphorylation of exclusively tyrosine residues in a variety of proteins that serve as substrates.