We have previously shown that natural killer (NK) cells play a role in protection against leishmaniasis. Furthermore, we have shown that NK cells in mononuclear cells derived from unexposed donors are induced to proliferate in vitro in response to leishmanial antigens. Since interleukin (IL)-12, a strong inducer of NK cells, acts on the early events in NK cells and T-cells, and is considered as an adjuvant for use in a potential antileishmaniasis antigen, we wished to investigate how this cytokine influences the in vitro Leishmania induced proliferative and cytokine response in healthy donors. We demonstrate that in an innate response to Leishmania antigen involving NK cells, a critical level of IL-12 is required to induce interferon (IFN)-gamma secretion below which, IL-10 is released in amounts which apparently inhibit IFN-gamma secretion and cellular proliferation. However, at higher IL-12 levels, there is simultaneous secretion of IFN-gamma and IL-10 as well as proliferation of cells. In a similar vein, exogenous IL-10 in turn inhibited IFN-gamma secretion as well as proliferation when used at low/medium concentrations, but at high concentrations this effect was abolished and replaced by the simultaneous detection of IFN-gamma, IL-10 and proliferation. The contribution of NK cells in cross regulation of these two very important immuneregulatory cytokines and the effect of exogenous IL-12 in a Leishmania driven response are discussed.