Japanese soybean paste miso, which has been reported to prevent gastric and mammary cancer and chronic nephritis, was demonstrated by electron spin resonance spectrometry using 5,5'-dimethyl-1-pyrroline-N-oxide as a scavenger of free radicals. Fifty mg/ml of miso scavenged 100% of 1,1-diphenyl-2-picrylhydrazyl radicals (3.9 x 10(15) spins/ml); 45 mg/ml quenched 92% of hydroxyl radicals (7.9 x 10(16) spins/ml); and 50 mg/ml quenched 50% of superoxide anion (6.7 x 10(16) spins/ml). In the system of rat cerebral cortex homogenate supplemented with 2 mM each of Fe2+ and ascorbic acid, 90% and 82% of the hydrogen and carbon-centered radicals having 1.7 x 10(13) spins/ml and 3.9 x 10(13) spins/ml, respectively, were quenched by 180 mg/ml of miso. The thiobarbituric acid-reactive substances, an index of lipid peroxidation in the brain, was inhibited by 10 mg/ml of miso. These results showed that miso acts as an antioxidant by scavenging free radicals.