The product of Rous sarcoma virus transforming gene has been shown to be a phosphoprotein of molecular weight 60,000, termed pp60src. Closely associated with pp60src is a protein kinase activity that results in the phosphorylation of tyrosine residues in a variety of polypeptides. At least one cellular protein has been identified as a substrate for pp60src. This protein with a molecular weight of 34,000 will be characterized as much as possible with regard to its normal functions in cells and efforts will be made to understand how this function(s) is altered by phosphorylation. Attempts will be made to characterize additional cellular substrates for pp60src as well. A new type of avian sarcoma virus has recently been characterized with transforming gene sequences unrelated to the Rous sarcoma virus transforming gene. Representatives of this group, PRCII and Fujinami, virus are also under investigation. Comparative biochemical studies will be conducted on their transforming gene products and on pp60src.