The capacity of human serum for inhibiting in vitro the membrane lipoperoxidation induced by a controlled system (ADP/NADPH + H+/Fe3+) was demonstrated. A concentration of 8 nmol of malondialdehyde was produced in 20 min in rat liver microsomes (1.5 mg of protein) after exposure to an induced lipoperoxidation mixture. Addition of 100 microliters (13.89 mg of protein) of human serum decreased malondialdehyde production nearly 50%. An increase of 25.97% of the inhibitory capacity of serum was obtained by the in vitro addition of 10 microliters/ml of vitamin E. Ten volunteers were supplemented with 400 mg of vitamin E and 1 g of vitamin C/daily for 2 weeks. Their serum inhibitory capacity increased in 12% (p < 0.05). The serum inhibitory capacity for microsomal lipoperoxidation is described herein, and we propose its utilization as an index to determine the individual nonspecific antioxidative defenses against free radical injury and lipoperoxidation in relation to exposure to air pollutants, tobacco smoke, and several acute and chronic diseases, including the hypoxia-reperfusion phenomena.