Giardia lamblia, a common pathogenic intestinal parasite of humans, was rapidly killed by exposure to normal human milk in vitro. The killing did not depend on secretory immunoglobulin A. Entamoeba histolytica, the dysentery amoeba, was also killed by normal human milk. Giardia-cidal activity cochromatographed with an unusual lipase that is present in the milk of humans but not of lower mammals. Human milk may play a protective role in infants exposed to this parasite.