TYROSINE protein phosphorylation is necessary for antigen receptor-mediated activation of T lymphocytes¹. This signal is generated at least in part by the Src-related tyrosine protein kinases p56^lck and p59^fynT (refs 2, 3). The activity of these two enzymes is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue^2,3. Recent studies suggest that this inhibitory phosphorylation may be caused by p50^csk (for C-terminal Src kinase), a tyrosine protein kinase which accumulates most abundantly in thymus and spleen^4–8. To investigate the function of Csk in T lymphocytes and characterize the processes regulating T-cell receptor (TCR) signalling, we examined the effects of overexpression of Csk on the physiology of an antigen-specific mouse T-cell line. We report here that p50^csk negatively regulates TCR-induced tyrosine protein phosphorylation and lymphokine production. This provides evidence for the involvement of Csk in the regulation of T-cell activation.