The levels of human erythrocyte superoxide dismutase activity were determined to establish a normal range classified by age. Blood samples were drawn from 11 little children aged 4 to 6, 47 young adults aged 20 to 35 who have no apparent diseases and 22 old people aged 70 to 100 living in a nursing home. The following results were obtained. 1, The distribution of SOD activity levels is 17.9±3.8(m+SD) Units/mgHb in children, 14.8+3.3 in young adults and 11.4±3.0 in old people. 2, The data show SOD activity decreases with age. Statistically the mean value of each group is significantly differes from one another.