The clinical course of giardiasis is variable, and serum antibodies do not appear to be protective. We propose that natural factors either produced by intestinal tissue, transported into the intestine, or ingested (ie, by breast-fed babies) might promote resistance to this disease. Human milk is very rich in secretory IgA (S-IgA) antibodies, as well as nonspecific antibacterial factors (eg, lactoferrin, lysozyme). Previous studies showed that Giardia lamblia trophozoites were killed by nonimmune human milk (NHM) in a time- and concentration-dependent manner. Removal of greater than 99% of the S-IgA from NHM did not decrease its Giardia-cidal activity. Thus, the killing was not antibody dependent. This is the first demonstration of nonimmune antiparasitic defenses in human milk. The present studies show that in the presence of NHM, trophozoites lost motility, swelled, and lysed. The Giardia-cidal activity (GCA) may be specific to human milk, since unheated cow's and goat's milk were virtually devoid of activity. Much, but not all, of the GCA was lost when NHM was heated or reacted with diisopropylfluorophosphate (DIFP), a specific esterase inhibitor. Activity of the major human milk lipase (BSL, bile salt-stimulated lipase, a fatty acid esterase) was lost after heat or DIFP treatment and was absent from cow's or goat's milk. The parasites were also killed by pure BSL. These studies suggest that BSL may be a heat-labile Giardia-cidal component of NHM.