Abstract Extensive genetic polymorphism of C3 was found in 81 sera from rhesus monkeys. Evidence was obtained for three alleles, rhesus C3S, C3F and C3F1, with gene frequencies of 0.66, 0.33 and 0.01. The variants were inherited as autosomal codominant traits. Polymorphism of Gc globulin was also found. Although most sera contained Gc globulin that formed a single band on electrophoresis, in a few sera this protein consisted of two bands, one of which was slower than the usual. These observations suggested that Gc globulin was controlled by two codominant alleles, rhesus Gc1 and Gc2, with frequencies of 0.975 and 0.025, respectively.