Observations are described on the occurrence of A, B, H, and Lewis blood group substances among Old World monkeys of the species Papio anubis, Papio cynocephalus, Mandrillus leucophaeus, Cynopithecus niger, Erythrocebus patas, Cercopithecus pygerythrus johnstoni, Macaca mulatta, Macaca irus, Macaca neme-strina, Macaca speciosa, and New World monkeys of the species Ateles marginatus, Ateles paniscus, Ateles fusciceps robustus, Ateles ater (Ateles paniscus chamek), Ateles cucullatus (Ateles geoffroyi grisescens), Cebus albifrons, Saimiri sciurea, and various species of marmosets. The significance of the findings and their application in taxonomy are discussed.