The protein tyrosine kinase activity of the cellular Src protein is negatively regulated by phosphorylation at tyrosine residue 527 (Tyr527). It has not been established whether this regulatory modification of Src is mediated by autophosphorylation or by another cellular protein kinase. The phosphorylation of a modified form of c-Src that lacks kinase activity was examined in mouse cells that do not express endogenous Src (because of the targeted disruption of both src alleles). Phosphorylation of the inactive form of Src on Tyr527 occurred to a similar extent in cells lacking endogenous Src as it did in cells expressing Src. Therefore, Tyr527 phosphorylation, and thus negative control of Src kinase activity, is mediated by another cellular protein tyrosine kinase.