Recombinant granulocyte/macrophage colony-stimulating factors (rGM-CSF) of mouse and human origins activated macrophages of the homologous species to inhibit the replication of the protozoan parasite T. cruzi. Activation could be induced with 10-100 ng/ml of rMu-GM-CSF, whether it was added before or after uptake of the parasite, in either adherent or suspension cultures. However, the degree of inhibition of parasite replication after exposure to rMu-GM-CSF was not as great as after treatment with rMu-IFN-gamma, and much more rMu-GM-CSF than rMu-IFN-gamma was required to achieve an equivalent antimicrobial effect. These results were mirrored by effects of the cytokines on enhancement of H2O2-releasing capacity in resident mouse peritoneal macrophages. In the latter tests, rMu-IFN-gamma and rHu-TNF-alpha afforded a 44-51-fold enhancement over the untreated control, with a 50% effective concentration (EC50) for rMu-IFN-gamma of approximately 0.05 ng/ml. Using rMu-GM-CSF or rM-CSF, enhancement of H2O2-releasing capacity was 14-15-fold over control, with EC50s of 1 and 14 ng/ml, respectively. However, peak enhancement of macrophage H2O2-releasing capacity was seen at least 24 h earlier with rMu-GM-CSF or rHu-M-CSF than with r-Mu-IFN-gamma or rHu-TNF-alpha. Thus, rMu-GM-CSF and rHu-GM-CSF displayed clear-cut macrophage-activating activity in vitro, but rMu-GM-CSF was less potent and less effective than rMu-IFN-gamma in the tests used.