The preexisting humoral immunity of four normal human beings to nine species of lower animals was assayed by conventional hemagglutination, leukoagglutination and cytotoxicity techniques. The specificity of the heterophile hemagglutinins was assessed by cross-absorption experiments. Preformed antibody activity was lowest against chimpanzee and baboon, and only slightly higher against sheep and goat. Hemagglutination titers against the several species closely paralleled the man-animal phylogenetic disparity. Hemagglutinins were found to have species-specific and broadly cross-reacting components.