The first blood group system to be defined in baboons by means of isoimmune agglutinating reagents is composed of five graded types: B4, B3, B2, B1and b, inherited, as postulated, by five allelic genes with decreasing dominance order. There are significant differences in distribution of the Bp blood groups between hamadryas baboons on the one hand, and cynocephalus and anubis on the other, which may serve as auxiliary taxonomic criteria. Antibodies of specificities related to the Bp graded antigens are found in the sera of presumably nonimmunized baboons, and also in the rhesus isoimmune antisera that detect one of the subtypes of the so-called Drh graded system of macaques. These findings point to ubiquity of the Bp or Bp-like antigens, and to the close serological relationship between Bp baboon and Drh macaque red cell antigens, perhaps as evidence of a common ancestral antigen which gave rise to a diverging set of graded subspecificities of baboon and macaque red cells, respectively.