The effects of 24 h supplementation of Caco-2 cells with carnosic acid and carnosol, and their activities against 5 microM oleic acid hydroperoxide (OAHPx)-mediated oxidative stress, were investigated. At 24 h of incubation, under nonstressed and stressed conditions, both compounds at 25, 50, and 100 microM supplement concentrations reduced catalase activity, whereas changes in glutathione peroxidase and superoxide dismutase activities varied depending upon the concentrations. Relative to control cultures, carnosic acid and carnosol reduced membrane damage by 40-50% when stressed by OAHPx. Carnosic acid and carnosol inhibited lipid peroxidation by 88-100% and 38-89%, respectively, under oxidative stress conditions. Both compounds significantly lowered DNA damage induced by OAHPx. Results of this study suggest that antioxidant activities of carnosic acid and carnosol could be partly due to their ability to increase or maintain glutathione peroxidase and superoxide dismutase activities.