19 of the 22 rhesus monkeys (Macaca mulatta) isoimmunized by us since 1965 produced hemagglutinating antisera. Seven contain the Erh specific antibody. Other antibodies determine the simian type blood factors Arh, Brh, Crh, Drh, Frh, Grh, Hrh, lrh, χrh; Yrh and Zrh in rhesus monkeys and by cross-reaction in another species, the crabeating macaques (Macaca fascicularis). The Erh is the most immunogenic blood factor-producing prozone with noninactivated sera. So far, no homologues of that factor could be demonstrated in man or in chimpanzees. Family studies indicate simple Mendelian dominance for several of the blood factors described.