We have shown previously that pp60c-src is a substrate for protein kinase C in vivo and that the target of protein kinase C phosphorylation in mammalian pp60c-src is serine 12. We now demonstrate that in addition to tumor promoters, all activators of phosphatidylinositol turnover that we have tested in fibroblasts (platelet-derived growth factor, fibroblast growth factor, serum, vasopressin, sodium orthovanadate, and prostaglandin F2 alpha) lead to the phosphorylation of pp60c-src at serine 12. In addition to stimulating serine 12 phosphorylation in pp60c-src, platelet-derived growth factor treatment of quiescent fibroblasts induces phosphorylation of one or two additional serine residues and one tyrosine residue within the N-terminal 16 kilodaltons of the enzyme and activates its immune complex protein-tyrosine kinase activity.