The effect of infection by prototypes from the three major species of Leishmania on the oxidative burst of human mononuclear phagocytes in culture was examined. The presence of intracellular parasites of the three species, L.major, L.donovani and L.mexicana decreased hydrogen peroxide (H2O2) and superoxide anion (O2-) production. This was particularly apparent when infected cells were compared to control monocytes following treatment with IFN-gamma. Nitroblue tetrazolium (NBT) reduction by monocytes was also decreased in infected cells. This morphological analysis of infected monolayers clearly showed that infected monocytes were incapable of reducing the dye as compared to uninfected cells. Decrease in NBT positive cells and production of H2O2 and O2- was related to the degree of infection of the monocyte monolayers. These results suggest that the presence of intracellular Leishmania amastigotes in mononuclear phagocytes decreases the oxidative burst and may contribute to parasite survival. Failure of phagocytes from patients with chronic granulomatous disease to kill these intracellular parasites also emphasized the importance of the oxidative burst for this function. Nevertheless, the consistent increase in leishmaniacidal effect attained after IFN-gamma treatment of the monocyte monolayers indicates that other non-oxidative mechanisms induced by this cytokine are also important in the killing of these intracellular parasites.