To understand host resistance to Giardia lamblia infection, we investigated the capacity of human peripheral blood effector cells to kill G. lamblia trophozoites. Peripheral blood was obtained from 12 healthy, uninfected individuals without detectable IgG anti-G. lamblia antibody or known exposure to G. lamblia. Isolated mononuclear leukocytes (MNL), granulocytes, lymphocytes, and monocytes were cultured 16 hr with G. lamblia that had been pulsed with (3H)TdR while in log growth. MNL from all 12 individuals were spontaneously cytotoxic for G. lamblia. Specific cytotoxicity ranged from 13% at an E:T of 3:1 to 56% at an E:T of 100:1. Release of 3H directly correlated with a decrease in viale G. lamblia. In contrast to the uniform cytotoxicity by mononuclear cells, granulocytes and lymphocytes had no spontaneous cytotoxic activity for G. lamblia. MNL depleted of phagocytic or adherent cells lost all cytotoxic activity. Also, G. lamblia induced nonspecific MNL cytotoxicity to unrelated target cells. Finally, cytotoxicity against G. lamblia could be inhibited by adding unlabeled Trichomonas vaginalis, suggesting that this may be a more general mechanism of host defense against extra-cellular protozoa. We conclude that human peripheral blood monocytes-macrophages are spontaneously cytotoxic for G. lamblia and that this may be a major mechanism of host defense against this parasite.