The erythrocytes of 350 pigtailed macaques (Macaca nemestrina) were examined for electrophoretic variation of hemoglobin and 26 enzymes. Seven enzymes showed variation in more than 1% of individuals: phosphoglucose isomerase, phosphoglucomutase-1, soluble NADP-dependent isocitric dehydrogenase, peptidase A, peptidase C, 2,3-diphosphoglycerate mutase, and acid phosphatase. Variation with lesser frequency was found in soluble glutamic-oxalacetic transaminase, phosphoglycerate kinase, lactic dehydrogenase, and hemoglobin. Only eight samples were tested for esterase D, and one of these had a variant phenotype. Enzymes with no clear variation were adenylate kinase, adenosine deaminase, phosphofructokinase, hexokinase, pyruvate kinase, glyceraldehyde 3-phosphate dehydrogenase, aldolase, phosphoglycerate mutase, phosphopyruvate hydratase (enolase), phosphoglucomutase-3, and superoxide dismutase. There was father-to-son transmission of PGI, PGM-1, peptidase C, 6PGD, 2,3-DPGAM, NADP-ICD, and acid phosphatase variants, suggesting that these loci are autosomal as in man.