The effects of a 30 mg/day beta-carotene supplement for 60 days on blood cell and serum antioxidant enzymes and selenium concentrations were examined in healthy adults. Serum beta-carotene concentrations increased significantly (P < 0.05) in response to supplementation. Forty percent of subjects exhibited hypercarotenemia of the skin after 30 days. There were no changes in the activity of red blood cell or leukocyte catalase activity, red blood cell copper,zinc-dependent superoxide dismutase activity or serum myeloperoxidase concentration in response to beta-carotene supplementation. Leukocyte superoxide dismutase activity decreased significantly (P < 0.05) at 30 and 60 days compared to baseline. Serum glutathione peroxidase concentration decreased significantly (P < 0.05) between baseline and days 45 and 60 of supplementation. Serum selenium and blood hemoglobin concentrations did not change during the study. Supplemental beta-carotene may alter the antioxidant capacity of plasma and/or blood cells in vivo.