Infective, stationary growth phase metacyclic promastigotes of Leishmania major were introduced to murine peritoneal macrophages and incubated with recombinant murine tumour necrosis factor (TNF) or with interferon gamma (IFN-gamma) in the presence of LPS. The leishmanicidal activity of macrophages was estimated by the visual counting of intracellular parasites as well as by the (3H) thymidine incorporation of the residual parasites. The culture supernatants were also collected for the estimation of nitrite (NO-2) and hydrogen peroxide (H2O2). The results showed that both (TNF) and (IFN-gamma) had a significant leishmanicidal effect, yet it was more pronounced with IFN-gamma. The production of NO-2 and H2O2 were also correlated with the increased leishmanicidal activity in the activated macrophages.