The mitogenic activation of resting T lymphocytes involves two distinct cellular events, the synthesis of the ultimate mitogen interleukin 2 and the synthesis and expression of receptors for it. In order to get more detailed information on the mechanisms associated with these activating steps (the effects of different stimuli, leading to activation of protein kinase C were investigated in human lymphocytes). The anti-T-cell receptor (TCR) and anti-CD3 monoclonal antibodies (BMA 031 and BMA 030, respectively), as well as the combination of the phorbol ester, TPA, with a calcium ionophore-induced interleukin 2 synthesis and subsequent proliferation in human peripheral blood lymphocytes. Incubation of cells with synthetic diacylglycerols and calcium ionophores proved to be effective in expression of high affinity interleukin receptors, no detectable amounts of interleukin 2 were, however, synthetized. When diacylglycerols were, however, added repetitively, interleukin 2 was also produced. Both anti-TCR/CD3 antibodies and TPA or DiC8 caused activation and translocation of protein kinase C from the cytosol to the plasma membrane. Significant differences, however, were observed between the time kinetics of the translocation of the enzyme. In plasma membranes of TPA-stimulated cells activation of protein kinase C was detectable up to 4 hr. In contrast, the highest specific activity of protein kinase C was measured in the plasma membranes after 15 min of DiC8 addition to cells. Anti-CD3 monoclonal antibodies activated protein kinase C in a biphasic manner. Shortly after binding of BMA 030 to the T cell antigen receptor/CD3 complex the activity of protein kinase C was increased in the plasma membrane, then it declined to control levels followed by a second long-lasting activation of the enzyme up to 4 hr. These results suggest different signal requirements for different activation steps. While for synthesis and expression of interleukin 2 receptors a short term activation of protein kinase C is sufficient, long-term activation of the enzyme is necessary for interleukin 2 synthesis in human lymphocytes.