The products of the blood group A and B genes, alpha-3-N-acetyl-D-galactosaminyltransferase (A-transferase) and alpha-3-D-galactosyltransferase (B-transferase) in 83 plasma samples of 24 species of nonhuman primates were investigated by enzymatic conversion of human group O red cells into A or B active cells. The conversions were demonstrated by the anti-A and anti-B agglutinability of the red cells. The plasma from group A and B anthropoid apes converted untreated group O human red cells into A and B active cells in the presence of UDP-N-acetyl-D-galactosamine and UDP-D-galactose, respectively. When papain-treated O red cells were used as an acceptor, A and B conversions by plasma glycosyltransferases of prosimians, New and Old World monkeys were detected. Most of the plasma from nonhuman primates contained A- or/and B-transferase activities that correspond to the A and B antigens expressed in the red cells. The A and B enzyme activities from nonhuman primates were almost equivalent to those from human A and B subgroups or variants. H antigen in the red cells of pig-tailed monkeys could be converted to A and B active ones by using plasma samples from man and nonhuman primates as enzyme sources. The optimal pH of A-transferases from Old World monkeys and human group A2 were 7.4 and 7.6, and from anthropoid apes and human group A1 they were 6.6 and 6.4, respectively. The pH optima of B-transferases from nonhuman primates and man distributed at 6.2-6.5.