Experiments were conducted to determine the antioxidant and prooxidant effects of beta-carotene, alpha-tocopherol and ascorbic acid on human lung cells at different oxygen (O(2)) tensions. Free radical initiator, 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH), was used to induce the cellular damage associated with lipid peroxidation, protein oxidation and DNA breaks. Under hypoxic conditions (0 torr O(2) tension) all compounds produced a concentration-dependent antioxidant effect. Mixtures of the three compounds exhibited greater protective affects than any individual compound. At 143 torr O(2) tension, all compounds exhibited concentration-dependent protective effects against AAPH-induced cellular lipid, protein and DNA damage. At 722 torr O(2) tension, cells exhibited a consistent increase in lipid peroxidation (isoprostane formation), protein oxidation (carbonyl formation) and DNA damage (p53 protein accumulation). beta-Carotene (1.5 microM) produced a prooxidant effect by promoting 12% isoprostane formation. Protein oxidation and DNA damage at 722 torr O(2) tension was not increased by beta-carotene; however, the antioxidant effect of beta-carotene was attenuated. The antioxidant effects of alpha-tocopherol, ascorbic acid, and mixtures of the three antioxidant compounds also were reduced by the high O(2) conditions. These results partially substantiate the hypothesis that the antioxidant and prooxidant effects of beta-carotene are dependent on O(2) tension and concentration of beta-carotene. Such findings may partially explain why selected populations, such as smokers, respond adversely when supplemented with beta-carotene.