A method is described for the estimation of red cell superoxide dismutase (erythrocuprein) and a normal range of activity established. It is likely that this enzyme is essential to the red cell for the detoxification of superoxide radicals, and plays a protective role similar to that of the glutathione-glutathione peroxidase system. It is suggested that superoxide dismutase deficiency may be an unrecognized cause of Heinz body hemolytic anemia. The separation of superoxide dismutase from hemoglobin by polyacrylamide gel electrophoresis is also described. Normal superoxide dismutase activity was measured in one case of Wilson's disease.