As reported previously, mouse peritoneal macrophages could be activated to kill intracellular trypomastigotes of Trypanosoma cruzi, the agent of Chagas' disease, in either of two ways: by immunizing and boosting the mice (3), or by culturing resident or inflammatory macrophages in spleen cell factor(s) (SCF) in vitro (2). Macrophages activated in vivo became less trypanocidal with time in culture, and cells activated in vitro lost trypanocidal capacity when CSF was removed (2). In the present study, the ability of macrophages to release H2O2 in response to phorbol myristate acetate (PMA) could be induced in vivo and in vitro, and reversed in vitro, in a manner correlating closely with changes in trypanocidal activity. Macrophages could be activated in vitro with SCF in a time-dependent and dose-dependent fashion, so that they released as much H2O2 as macrophages activated in vivo. The sensitivity of epimastigotes and trypomastigotes to enzymatically generated H2O2 suggested that the generation of H2O2 by activated macrophages could be plausible explanation for their trypanocidal activity. Of the biochemical correlates of macrophage activation reported to date, increased ability to release H2O2 seems most closely allied to enhanced capacity to kill an intracellular pathogen.