Interaction of the interleukin 2 receptor (IL-2R) beta chain with the lymphocyte-specific protein tyrosine kinase (PTK), p56lck, has led to the speculation that p56lck participates in growth signal transduction. Although activation of T cells with interleukin 2 (IL-2) results in the activation of p56lck, accumulating data support the notion that Lck does not play an essential role in mitogenic signal delivery from the IL-2R. Since this src-related PTK has been shown to enhance TCR/CD3-mediated T cell responsiveness, here we investigated whether activation of Lck by IL-2 could contribute to enhance TCR/CD3-mediated T cell functions. This was achieved by using human CD4(+)-cloned T cells and comparing the effects of IL-2 on p56lck kinase activation and cytokine production. Results show that p56lck kinase activity increased as early as 1 min, reached a maximum within 5 min and decreased within 60 min after IL-2 stimulation. Such treatment with IL-2 also resulted in enhancing T cell responsiveness to CD3+PMA stimulation, as assessed by IL-2 and IFN-gamma secretion and by T cell proliferation. This increase of T cell functions was correlated with IL-2-induced p56lck activation in both dose-response and time-course experiments. Taken together these results strongly suggest that activation of Lck by IL-2 may play a role in regulating CD3-mediated T cell functions.