Protein tyrosine kinases participate in the transduction and modulation of signals that regulate proliferation and differentiation of cells. Excessive or deregulated protein tyrosine kinase activity can cause malignant transformation. The catalytic activity of the T cell protein tyrosine kinase p56lck is normally suppressed by phosphorylation of a carboxyl‐terminal tyrosine, Tyr‐505, by another cellular protein tyrosine kinase. Here we characterize a human cytosolic 50 kDa protein tyrosine kinase, p50csk, which specifically phosphorylates Tyr‐505 of p56lck and a synthetic peptide containing this site. Phosphorylation of Tyr‐505 suppressed the catalytic activity of p56lck. We suggest that p50csk negatively regulates p56lck, and perhaps other cellular src family kinases.