The relative contribution of endogenously secreted lymphokines, TNF-alpha and IL-2, to T cell activation has been studied. Interestingly, a neutralizing anti-TNF-alpha mAb strongly inhibited the proliferation, IL-2R alpha expression, and activation of the nuclear factor (NF)-kappa B induced on human purified T lymphocytes by an immobilized anti-CD3 mAb (imm.anti-CD3). Furthermore, the addition of exogenous TNF-alpha to T cells activated by imm.anti-CD3 or phorbol esters strongly stimulated all those activities. Similarly, neutralizing anti-IL-2R alpha Abs inhibited the cell proliferation and the IL-2R alpha expression induced by imm.anti-CD3, whereas exogenous addition of IL-2 enhanced both activities. However, exogenous addition of IL-2 did not affect NF-kappa B activation. Cyclosporin A, which prevented lymphokine mRNA transcription, inhibited more than 90% of the IL-2R alpha and NF-kappa B levels induced by TCR/CD3-mediated activation. Exogenous TNF-alpha and IL-2 were equally important to the partial recovery of this IL-2R alpha inhibition, and high doses of both lymphokines together completely abolished the inhibitory effect of cyclosporin A. However, only TNF-alpha (but not IL-2) efficiently recovered the nuclear NF-kappa B levels. Those results indicate that the autocrine secretion of TNF-alpha and IL-2, and not the primary stimulus itself, contribute mostly to the regulation of IL-2R; thereby playing a very important role in quantitative control of T cell proliferation and leaving to the specifically TCR/CD3-derived signals the triggering role in T cell activation.