Nitric oxide (NO) produced by cytokine-treated macrophages and hepatocytes plays a vital role in protective host responses to infectious pathogens. NO inhibits iron-sulfur-dependent enzymes involved in cellular respiration, energy production, and reproduction. Synthesis of L-arginine-derived nitrite (NO2-), the oxidative end product of NO, directly correlates with intracellular killing of Leishmania major, an obligate intracellular protozoan parasite of macrophages: the level of NO2- production is a quantitative index for macrophage activation. The competitive inhibitor of NO synthesis, monomethylarginine (NGMMLA), inhibits both parasite killing and NO2- production. For Leishmania, the parasite itself participates in the regulation of this toxic effector mechanism. This participation is mediated by parasite induction of tumor necrosis factor alpha (TNF alpha), an autocrine factor of macrophages: NO synthesis by interferon-gamma (IFN-gamma)-treated cells can be blocked by monoclonal antibodies to TNF alpha. NO production by IFN gamma-treated hepatocytes is of special interest in malaria infections: sporozoite-infected hepatocytes kill the intracellular malaria parasite after treatment with IFN gamma; this killing is inhibited by NGMMLA.