Distribution of the human-type and the simian-type blood groups in rhesus [Macaca mulatta], crab-eating [M. fascicularis], bonnet [M. radiata], pig-tailed [M. nemestrina] and stump-tailed macaques [M. arctoides] revealed significant similarities and differences among these species. Human-type A-B-O blood groups cut across taxonomic lines and seem of less value for taxonomic purposes than the simian-type blood groups detected by cross-reacting isoimmune rhesus monkey sera.