The effects of gamma interferon (IFN-gamma) on P388D1 cell or mouse resident peritoneal macrophage association (i.e., binding and internalization) with the protozoan Trypanosoma cruzi were studied, as well as the effects of this lymphokine on intracellular parasite killing. Incubation of either type of cell with a conditioned medium containing IFN-gamma and traces of interleukin 2 markedly increased the capacities of the cells to associate with virulent blood forms of T. cruzi, as evidenced by significant increases in both the proportion of parasite-associated cells and the number of parasites associated with the cells. Three lines of evidence pointed to IFN-gamma, and not interleukin 2, as the lymphokine responsible for the noted effect. First, a conditioned medium containing interleukin 2 but not IFN-gamma failed to enhance P338D1 cell-parasite association. Second, treatment of the IFN-gamma preparation at pH 2 to selectively inactivate IFN-gamma reduced its enhancing effect. Third, recombinant IFN-gamma, devoid of other lymphokines, also enhanced parasite association with P388D1 cells. Incubation of P388D1 cells with IFN-gamma for 24, 48, or 72 h increased cell association with T. cruzi, whereas a 12-h incubation period was insufficient, suggesting that IFN-gamma triggered time-dependent cellular events leading to the enhancement. Treatment of mouse resident peritoneal macrophages with the IFN-gamma-containing conditioned medium also increased the capacity of these cells to kill internalized trypanosomes. P388D1 cells, which showed minimal or no cytotoxicity after mock treatment with medium, displayed cytotoxicity after incubation with the IFN-gamma-containing conditioned medium; similar results were obtained with recombinant IFN-gamma. Catalase prevented parasite killing by P388D1 cells, indicating that H2O2 mediated the cytotoxicity. These results, underscoring the regulatory effects of IFN-gamma on macrophage-parasite interactions, suggest a possible role for this lymphokine in the mechanisms of host defense active against T. cruzi infection.