Stimulation of quiescent T lymphocytes to proliferate involves a complex series of events both between and within cells. At least 70 genes are known to be induced or activated from the time of the initial stimulation until DNA synthesis. While some of these gene products, e.g., interleukin-2 (IL-2) and IL-2 receptors, are required for proliferation, others, e.g., gamma-interferon and colony-stimulating factor, are ancillary to activated T cell function. Several biochemical signal transductions are among the early events. One of the earliest is phospholipase C-mediated hydrolysis of phosphatidylinositol leading to release of diacylglycerols and inositol phosphates, which in turn activate protein kinase C and elevate intracellular free calcium levels. The discovery that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) both enhances proliferation and activates protein kinase C strengthens the evidence for a general role of protein kinase C in proliferation. Yet, the exact consequences of stimulation of protein kinase C in regard to specific proliferation proteins is still not clear. In this study, we present evidence that protein kinase C activation is directed to production of IL-2 but not to IL-2 receptors. Under conditions of TPA treatment in which protein kinase C was chronically reduced in T lymphocytes, IL-2 production was greatly depressed as were the level of IL-2 mRNA and [3H]thymidine incorporation. In contrast, these cells still expressed high affinity IL-2 receptors and proliferated when endogenous IL-2 was added. Because neither phosphatidylinositol metabolism nor Ca2+ flux was affected, the block appeared to be mediated directly or indirectly through protein kinase C.