Serological studies on the bloods of thirty-six species of lower monkeys have shown that there exists a correspondence between the distribution of a certain hemagglutinogen and the place of the species in the zoological system. In twelve species of seven genera of Platyrrhina (New World monkeys) and six species of the genus Lemur a factor similar to the human isoagglutinogen B was present; in eighteen species of four genera of Cercopithecidae (Old World monkeys) it was absent, although the latter are more closely related to man than the former. It would seem from our findings that a genus, perhaps even a family, of animals may be characterized by a special serological factor. The factor found in the lower monkeys is not identical with the one existing in the erythrocytes of the anthropoid apes and man.