Protein-tyrosine kinases (PTKs) play an integral role in T cell activation. Stimulation of the T cell antigen receptor (TCR) results in tyrosine phosphorylation of a number of cellular substrates. One of these is the TCR ζ chain, which can mediate the transduction of extracellular stimuli into cellular effector functions. We have recently identified a 70 kd tyrosine phosphoprotein (ZAP-70) that associates with ζ and undergoes tyrosine phosphorylation following TCR stimulation. Here we report the isolation of a cDNA clone encoding ZAP-70. ZAP-70 represents a novel PTK and is expressed in T and natural killer cells. Moreover, tyrosine phosphorylation and association of ZAP-70 with ζ require the presence of src family PTKs and provide a potential mechanism by which the src family PTKs and ZAP-70 may interact to mediate TCR signal transduction.