Four isoimmunized baboons each produced isoantibodies defining a number of blood factors of baboon blood of which two, Ap and Bp, have been most intensively studied. The two blood factors determine the AP-BP blood group system which, judging from its serological behavior, may be the baboon analogue of the human M-N system and the chimpanzee V-A-B system. Tests for AP-BP types of 592 baboons showed striking differences in the distributions of the four types among the four sub-species, Papio cynocephalus, Papio anubis, Papio ursinus (South Africa) and Papio papio (Senegal). The baboon AP-BP types could also be demonstrated by tests on the red cells of geladas (Theropithecus gelada). If one assumes inheritance by multiple allelic genes, then the existence of only a single gene Op need be invoked for Papio ursinus, three alleles Op, Ap and Bp for Papio cynocephalus and Papio anubis, but four alleles for Papio papio including an allele, very frequent in that subspecies, which determines an agglutinogen having both blood factors Ap and Bp.