Genetic polymorphism of the third component of human complement and its breakdown products has been detected in human serum by high-voltage starch-gel electrophoresis. Six phenotypes were observed in a study of 113 randomly chosen Caucasians. Their inheritance is controlled by four codominant alleles at an autosomal locus. The gene frequencies in this study were C3(1), 0.21; C3(2), 0.77; C3(3), approximately 0.01; and C3(4), approximately 0.004.