Confluent cultures of human skin fibroblast lines established from fetal and postnatal donors were exposed to a broad range of oxygen tensions (10-600 mmHg) for 1 wk; superoxide dismutase (SOD) activity was subsequently determined. Hyperoxia increased SOD activity slightly in postnatal lines but not in fetal lines. The magnitude of the increase in postnatal lines was not significant. Fetal lines exhibit only about one-fifth the SOD activity observed in postnatal lines. The results indicate that, while development-associated changes in SOD do occur in human cells, these alterations do not result from variations in ambient oxygen tension.