Eighty-one black-handed tamarins from the Tucurui region were tested for human type ABO blood groups by salivary inhibition tests. Eleven belonged to the A group, 45 to B, and 25 to AB. The serum samples were tested for the presence of agglutinins having specificities like those of humans. The ABO system appeared to be polymorphic, with three alleles occurring at the following frequencies: A = 0.26, B = 0.66, and O = 0.08. The observed distribution fitted the expected on the basis of Hardy-Weinberg equilibrium.