Antioxidant enzymes (catalase, superoxide dismutase and glutathione peroxidase) have been injected into human fibroblasts exposed to 2 atm O2 in order to test if the threshold of oxidative damage versus antioxidant defenses could be modulated and if the damage remains reversible beyond the threshold. Cell damage was estimated by thymidine incorporation and cell survival curves. The proportion of dividing cells, measured by thymidine incorporation, rapidly decreased after O2 incubation: no cells could divide after 15 h of hyperoxia. However, cells incubated for a short time and injected with a high concentration of any of the three enzymes divided like non-oxygen-incubated cells; the enzymes could protect the cells against their loss of division potential. However, when cells were incubated for a longer period and/or when the injected enzyme concentration was lower, cells were either less or not protected and could no longer divide. These results suggest the presence of a threshold for the oxidative damage which cannot be totally repaired and which impairs the cell division; this threshold can, however, be modulated by supplementation of antioxidant enzymes, glutathione peroxidase being the most efficient.