To understand murine host responses to extracellular protozoa, the capacity of peritoneal macrophages to exhibit Cytotoxicity for [3H]thymidine-labeled Giardia lamblia trophozoites was investigated. Resident peritoneal macrophages from C3H/HeN mice expressed spontaneous cytotoxicity for G. lamblia in a manner that was dependent on both time and effector cell number; this cytotoxic activity was increased with cells elicited by an intraperitoneal injection of thioglycollate. In contrast, spontaneous Cytotoxicity for G. lamblia by resident and thioglycollate-elicited peritoneal macrophages from C3H/HeJ mice was markedly reduced. In the presence of anti-G. lamblia serum (ADCC), however, peritoneal macrophages from both C3H/HeN and C3H/HeJ mice exhibited striking augmentation of their cytotoxic activity for G. lamblia to equivalent levels. We conclude that macrophages from C3H/HeJ mice express defective spontaneous Cytotoxicity but normal ADCC for the extracellular protozoan parasite, G. lamblia. The dissociation between the expression of these two effector cell functions suggests that macrophage spontaneous Cytotoxicity and ADCC for extracellular protozoa are mediated by separate macrophage functions. © 1984.