The relatedness of owl monkey, squirrel monkey, marmoset and human pituitary growth hormone was measured immunologically by quantitative complement fixation with rabbit antiserum to human growth hormone. Complement-fixation curves obtained with antihuman growth hormone and owl monkey, squirrel monkey and marmoset pituitary extracts were similar. However, the complement-fixation curves obtained with the 3 nonhuman primate growth hormones were qualitatively different from complement-fixation curves obtained with human growth hormone. Serologic specificities of antibodies to human growth hormone produced in the rabbit, guinea pig, rat and human were also examined. Guinea pig, rat and human antibodies, which did not fix complement directly with primate growth hormones, were detected by inhibition of complement fixation. The specificities of these antisera were measured by the extent of their cross-reactions with 8 growth hormones obtained from 3 different superfamilies of primates: Hominoidea, Cercopitheco idea and Ceboidea. All the antisera (rabbit, guinea pig, rat and human) to human growth hormone cross-reacted less effectively with growth hormones from the Ceboidea than with growth hormones from the other 2 groups of primates. The extent of the cross-reactions of guinea pig and rat antibodies, measured by inhibition of complement fixation, was similar to the extent of the cross-reactions of rabbit antibodies, measured by direct complement fixation. Human antibodies, however, showed a considerably greater specificity for growth hormones of the Hominoidea. The degree of relatedness of nonhuman primate growth hormones to human growth hormone correlated with primate phylogeny when measured by direct complement fixation with rabbit antihuman growth hormone or by inhibition of complement fixation with guinea pig, rat or human antihuman growth hormone. (Endocrinology79: 615, 1966)