ZAP‐70 is a protein tyrosine kinase (PTK) required for T‐cell development and T‐cell antigen receptor (TCR) function. ZAP‐70 is associated with the phosphorylated antigen receptor and undergoes tyrosine phosphorylation following receptor activation. We demonstrate here that tyrosine phosphorylation of ZAP‐70 results in an increase in its catalytic activity and that this activation is mediated by the phosphorylation of tyrosine residue 493 by the src family of PTKs. The activity of baculoviral expressed ZAP‐70 was up‐regulated 10‐fold when ZAP‐70 was co‐infected and phosphorylated by the src family PTK, lck. Mutation of Y493 alone abrogated the ability of ZAP‐70 to be activated by lck. Moreover, we demonstrate that phosphorylation of Y493 and activation of ZAP‐70 is required for antigen receptor‐mediated induction of interleukin‐2 (IL‐2) secretion in lymphocytes.