It was reported previously that cloned Th1 cells, but not Th2 cells, raised to malaria antigens, produce nitric oxide (NO) when activated with specific antigen or mitogen. Furthermore, NO inhibits the proliferation of, and production of interleukin-2 (IL-2) and interferon-gamma by, Th1 but not Th2 cells. By dose-response analysis, I demonstrate here that Th1 cells produce optimal levels of IL-2 and a proliferative response, and no detectable NO, when stimulated with relatively low concentrations of antigen or mitogen in vitro. As the antigen/mitogen increased, however, high levels of NO were produced, accompanied by a concomitant reduction in IL-2 secretion and T cell proliferation. At the highest concentrations of antigen/mitogen examined, addition of recombinant IL-2 reversed the NO-mediated downregulation of T cell proliferation. These results suggest that NO may serve as a self-regulatory molecule preventing the over-expansion of Th1 cells. At the other extreme, exogenous IL-2 may act to counter-regulate the suppressive effect of high concentrations of NO on Th1 cell proliferation, thereby maintaining homeostasis.