Recombinant murine IFN-gamma (rMu-IFN-gamma) was demonstrated to be a potent in vivo activator of mouse peritoneal macrophages to kill Trypanosoma cruzi in vitro and to be capable of conferring protection against death from acute T. cruzi infection. Following i.p. injections of rMu-IFN-gamma, resident peritoneal macrophages were cultured and infected with T. cruzi in vitro. Numbers of intracellular parasites were determined at different times thereafter. Ten or 100 micrograms (1 microgram = 6.5 X 10(5) U) of Mu-IFN-gamma, injected both 24 and 4 h before macrophage harvest, induced up to 99% inhibition of T. cruzi. One microgram of rMu-IFN-gamma was not effective under these conditions. In vitro inhibition of T. cruzi by peritoneal macrophages occurred by 24 h after infection and continued until at least 120 h after infection. There were no significant differences in initial parasite uptake by macrophages from IFN-gamma-treated or control mice, indicating that the rMu-IFN-gamma induced parasite killing. One i.p. dose of 10 micrograms was as effective as two doses if the single injection was given 24 h before macrophage harvest. In subsequent experiments, mice were given multiple injections of 10 micrograms rMu-IFN-gamma beginning 24 h before or 2 h after infection with virulent T. cruzi. Mice treated with rMu-IFN-gamma had significantly lower parasitemias and decreased morbidity compared with control mice. Proliferative responses to Con A and antibody responses to SRBC were not significantly lowered in IFN-gamma-treated mice, in contrast to untreated infected controls. All of the IFN-gamma-treated mice survived acute T. cruzi infection, whereas 100% of saline-treated infected mice died. It was demonstrated in this study that rMu-IFN-gamma activated mouse macrophages in vivo to kill T. cruzi and that rMu-IFN-gamma significantly reduced morbidity and immune suppression, and eliminated mortality resulting from acute infection with this parasite.