1. Activities, contents and true specific activities of superoxide dismutase (SOD) were determined in human erythrocytes of 105 normal healthy subjects.

2. The mean SOD activity assayed by the inhibition of xanthine autoxidation was 11·0 × 103 units/g of haemoglobin (Hb).

3. The mean SOD content assayed by an immunochemical method was 456 μg/g of Hb.

4. Both activity and content of SOD showed normal distributions, and no significant variations with regard to sex and age were detected.

5. A highly positive correlation between activities and contents of SOD was observed in normal healthy subjects (r = 0·77, P < 0·001).

6. True specific activity was calculated from levels of activity and content of SOD. The mean true specific activity of SOD in human erythrocytes was 23·7 units/μg of SOD.

7. There was no significant difference in true specific activity between young and old subjects.

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