It has been postulated that superoxide dismutase (SOD) protects cells from free radical-induced damage. In these experiments SOD specific activity was measured as established human diploid cell lines from various donor ages progressed through their in vitro lifespan. Significant elevations in activity occurred during the in vitro lifespans of cells from fetal and newborn donors, but no change in activity was detected during the lifespan of cells from an adult donor. In addition, a direct relationship between enzyme activity and donor age was detected with the following relative activities: adult greater than newborn greater than fetal. The possible relationship between these findings and the free radical theory of aging is discussed.