In this study, we report that trypomastigotes of T. cruzi coated with eosinophil peroxidase (EPO) become sensitized to killing by normal macrophages that are unable to kill uncoated organisms. EPO bound to the surface of the organisms without affecting their extracellular viability. The intracellular killing of EPO-coated trypomastigotes could be inhibited by catalase and azide, suggesting that toxicity was mediated through the small amounts of hydrogen peroxide generated by the phagocytic event in normal macrophages and the peroxidatic activity of EPO. EPO-coated organisms could be killed in a cellfree system by the addition of H2O2 and either iodide, bromide, or chloride. Omission of H2O2 decreased but did not prevent the killing of trypanosomes by the cellfree system and this residual toxicity was abolished by catalase. This suggests that H2O2 generated by trypanosomes contributes to the death of EPO-coated organisms. EPO-coated organisms could also be killed extracellularly when exposed to normal macrophages at high parasite to cell ratios or when a high phagocytic load of another particle was given simultaneously. This effect could be inhibited by both azide and catalase, but not by superoxide dismutase. This suggests that enough H202 is released by phagocytosis of a high number of organisms to generate toxic concentrations of H2O2 outside the confines of the vacuolar system.