We have previously shown that during an infection with Leishmania major, susceptible BALB/c mice, as opposed to mice of a resistant strain (C57BL/6), are primed by lipopolysaccharide for the production of high levels of tumor necrosis factor-alpha (TNF-alpha) which is known to be a potent macrophage (M phi) stimulator in other parasitic diseases. In the present study we investigated whether TNF-alpha activates M phi for killing of L. major parasites. In the absence of interferon-gamma (IFN-gamma) or lipopolysaccharide, TNF-alpha (0.025-25,000 U/ml) failed to activate peritoneal exudate M phi from BALB/c mice for killing of L. major amastigotes. In the presence of suboptimal doses of IFN-gamma (5 or 10 U/ml), however, TNF-alpha mediated a rapid elimination of intracellular parasites, which was highly significant compared to IFN-gamma alone. The combination of TNF with interleukin 4, in contrast, was inactive in this respect and allowed survival of intracellular parasites. From these data we conclude that the presence of IFN-gamma is crucial for TNF-alpha-mediated killing of L. major parasites by M phi. Disease progression in susceptible mice therefore seems to be a consequence of a deficiency of IFN-gamma and a predominance of interleukin 4 rather than the result of an excess amount of TNF-alpha.