Two isozymes of superoxide dismutase, A and B, were found in extracts of human organs, tissues and cell cultures. Differential centrifugation of cell homogenates revealed that isozyme A was localized in the soluble phase and isozyme B in fractions enriched in mitochondria. Isozyme B was lacking in erythrocytes and isozyme A in polymorphonuclear leucocytes. All other specimens showed both isozymes although in variable proportions. Evidence was presented suggesting that isozymes A and B are controlled by genes at separate gene loci. The occurrence of hybrid enzyme in a heterozygous phenotype of isozyme A suggests that the isozyme is a dimer made up of two identical polypeptide subunits.