Eighteen different primate species have been typed for Gm factors present at the human γ(2b) and γ(2c) loci. Gm factors are absent in lower primates. The Gm factors, associated with γ(2C) molecules, namely, Gm(s), (c^5), (b^0) evolve first in the Cercopithecoidea species. Gm(z) occurs together with Gm(a) on human γ(2b) molecules, but in baboon sera Gm(z) occurs in the absence of Gm(a). The simultaneous occurrence of Gm(a) and (z) in the same sera is established in Hominoidea. Gm(b^3) is present only in gorillas, orangutans, and chimpanzees, while Gm(b4) and (c^3) was detected in chimpanzees. Polymorphism for the factors studied exists only in orangutans and chimpanzees. The data show a close correlation between taxonomic classification and the evolution of Gm factors.