ABSTRACT Quantitative tests show that an antiserum for an individual of one species of the Old World monkey family Cercopithecidae may react no more strongly with the blood of another individual of the same species than it does with the blood of monkeys belonging to other species or genera of the same family. They also show that in this family of Primates interspecific precipitin responses may be no stronger than intergeneric ones. Furthermore, the specific and generic serum interrelationships of these monkeys may be no closer than their interfamilial serum relationships to the chimpanzee. The latter relationship appears, however, to be closer than the monkeys’ interfamilial serum relationship to man. Human serum nevertheless does give a group reaction to anti-Old World monkey serum, whereas such a response is not given either by the brown capuchin, a New World monkey, or by the lemur, Perodicticus potto. These findings support the view, developed both by Erhardt (1929) and Manwaring (1934), that the serum-precipitin test is of limited value in tracing phylogenetic relationships.