Two categories of blood groups, human-type and simian-type, occur in apes and monkeys and can be routinely tested by methods established for grouping human blood. Abundant data have been obtained on blood groups of chimpanzees, baboons and macaques. Studies of populations of animals, both feral and kept in captivity, resulted in the definition of a number of erythrocyte antigens, some of which fall into separate blood group systems. Two complex chimpanzee blood group systems, V-A-B-D and R-C-E-F systems, proved to be counterparts of the human M-N-S and Rh-Hr blood group systems, respectively. Two graded blood group systems were defined in Old World monkeys: the Drh system of macaques and the Bp system of baboons, both linked by at least one specificity shared by either of the blood group systems. With established modes of inheritance, blood groups of apes and monkeys become useful chromosomal markers in genetic studies of primates as well as important tools for taxonomists. Serology of some of the blood groups suggests their practical importance in breeding of primates as well as in clinical and experimental problems calling for transfusion of blood or transplantation of organs and tissues.