The possible inhibitory effects were investigated for three isoflavones: sophoraisoflavone A, and licoisoflavones A and B, isolated from Sophora mooracrotiana Benth ex Baker, on lipid peroxidation by superoxide anion. They inhibited the production of lipid peroxidation each by superoxide, anion and the generation of superoxide anion by the xanthine-xanthine oxidase system. Their effects were similar to superoxide dismutase as a superoxide anion scavenger. These results demonstrate that these isoflavones have inhibitory effects on oxidative stress.