Hematologic and coagulation studies were determined in 5 species of nonhuman primates, dogs, rabbits and man. The coagulation studies revealed high concentrations of factor VIII and XII in the primates. Although only minor differences were found among the species, these should be considered in choosing an animal model to simulate man. We have chosen the baboon because of its closest similarity to man, ready availability, hardiness, and increasing utility in medical and surgical research.