To determine the role of interferon-gamma (IFN-gamma) in the activation of macrophages to kill Entamoeba histolytica trophozoites in vitro, C57BL/6 mice were injected with various doses of recombinant IFN-gamma (rIFN-gamma) by either the intravenous, intraperitoneal or intramuscular routes. Mice were treated with doses of rIFN-gamma ranging from 10(1) to 10(5) units. Twenty hours later, peritoneal macrophages were harvested from the treated animals. Macrophage monolayers were prepared and their in vitro cytotoxic activity against a virulent strain of E. hystolytica (IP:0682:1) was determined. Amoebicidal activity was determined by counting the number of dead trophozoites by Trypan Blue exclusion in cultures containing macrophages and amoebic trophozoites which were incubated together for 4 h. Both intravenous and intraperitoneal treatment resulted in the recovery of macrophages from the peritoneal cavity which exhibited amoebicidal activity in vitro. Peritoneal macrophages harvested from mice that had been treated intraperitoneally or intravenously with rIFN-gamma, however, showed significantly more amoebicidal activity in comparison to macrophages harvested from animals treated intramuscularly. There was a dose dependent relationship between the concentration of rIFN-gamma used to activate macrophages in vivo and the number of dead trophozoites in vitro. In addition, these results confirm our previous observations that treatment in vitro with rIFN-gamma can activate murine peritoneal macrophages to kill amoebic trophozoites.