Superoxide dismutase (SOD), widely distributed in aerobic organisms, catalyzes dismutation of the superoxide free radical, O2-, to oxygen and hydrogen peroxide and apparently protects against oxygen toxicity. In human erythrocytes, O2- arises from autoxidation of oxyhemoglobin and SOD activity is copper-dependent. Since human erythrocyte copper content has been reported to decline in the fifth decade of life, we investigated the age dependence of human erythrocyte SOD activity. The mean SOD activity, assayed by inhibition of epinephrine autoxidation, was 415 +/- 66 units/g cells or 50 +/- 11 units/mg non-Hb protein. No significant difference was observed between young and old adults, and no correlations were detected with sex, state of health of the donor, or with blood hemoglobin content. The lack of general decline of SOD activity with age narrows the possible mechanisms for an involvement of O2- in senescence. SOD may yet decline in other longer-lived tissues or, as suggested by Fridovich, a constant low level of damage may be caused by imperfect scavenging of O2- by SOD. If such a mechanism is operative, it appears not to affect synthesis of SOD in erythrocyte precursor cells into the eighth decade of human life.