Two allelic isozymes of adenine phosphoribosyl transferase (APRT) were detected by starch gel electrophoresis of baboon hemolysates. Extensive family data verified autosomal codominant inheritance. The gene frequencies of five subspecies of baboons differed significantly. The activity of erythrocyte APRT is sufficiently high to enable the use of this enzyme as a sensitive marker for assessing chimerism in research involving bone marrow transplantation.