Mice (3–5 per group) were injected intraperitoneally with sterile thioglycollate solution and 4–5 d later, the animals were killed and their peritoneal cells were isolated and pooled. The populations were then pretreated for 2 h with murine IFN-γ (100 U/ml) in the presence or absence of 2.5 mM l-NMMA. Cultures were subsequently infected with RH tachyzoites and 24 h later pulsed with ³H-uracil. Incorporated radioactivity was next determined and expressed as mean CPM ± SE for triplicate cultures. Percentage killing was then calculated using the formula indicated in Materials and Methods. The experiments shown are representative of five performed.