Polymorphism of human C'3 has been defined by prolonged agarose electrophoresis of fresh serum. At least four, and probably five, alleles have been identified by the electrophoretic mobilities of gene products. Inheritance of three alleles, F(1) F, and S, is consistent with the autosomal condominant type. The inheritance of S(1) is probably codominant and that of F(0.8) is not known. Of the 15 phenotypes predicted by these alleles, eight have been observed. The allotypes appear to differ in net surface charge at pH 8.6, but show no obvious differences in complement activity, in molecular size, or in binding of Ca(++). The concentrations of the two gene products in serum from all known heterozygotes are approximately equal. The S gene is most common in the three major races of man. The F gene is relatively common in Caucasians, less common in American Negroes, and relatively uncommon in Orientals.