The effect of b-carotene on protein oxidation was examined under different oxygen (O 2 ) tensions and with other antioxidants: a-tocopherol, ascorbic acid, and mixtures of antioxidants. Human serum albumin (HSA) was incubated with 2,2%-azobis (2-amidinopropane) dihydrochloride (AAPH) to induce protein oxidation (carbonyl formation), under 15, 150, and 760 torr of O 2 tension. Antioxidant activity was related to O 2 tension, antioxidant concentrations and interaction between mixtures of antioxidants: (1) Under 15 torr of O 2 , incubating HSA with AAPH, 1.6 mM b-carotene, 80 mM a-tocopherol, 160 mM ascorbic acid, and mixtures (0.1 mM b-carotene, 5.0 mM a-tocopherol and 10 mM ascorbic acid) resulted in 24, 29, 39, and 44% reduction of carbonyl formation, respectively. (2) Under 150 torr of O 2 tension, the antioxidant effect of b-carotene was decreased by 4% but increasing O 2 tension did not diminish the antioxidant effects of a-tocopherol, ascorbic acid, or antioxidant mixtures. (3). Under 760 torr of O 2 tension, adding 1.6 mM b-carotene resulted in 26% more carbonyl formation. (4) Under 760 torr of O 2 tension, the antioxidant effect of ascorbic acid was decreased 32% compared to what was observed at 150 torr of O 2 tension. Changes in O 2 tension had no effect on the antioxidant effect of a-tocopherol. The mixture of antioxidants inhibited carbonyl formation by 37% and was 7% less effective than that of 15 and 150 torr of O 2 tension. High concentration of b-carotene produces more protein oxidation in the presence of high O 2 tension by a prooxidant mechanism. Mixtures of b-carotene, a-tocopherol, and ascorbic acid provided better protective effects on protein oxidation than any single compound.