Incubation of normal erythrocytes with sodium dithionite resulted in the formation of Heinz bodies, but incubation with sodium metabisulfite did not. Addition of superoxide dismutase to the incubation medium increased the formation of Heinz bodies by sodium dithionite. Addition of catalase to suspensions of erythrocytes in the presence and absence of superoxide dismutase inhibited the formation of Heinz bodies. These findings indicate that hydrogen peroxide, not superoxide, is the active oxidant in Heinz body formation.