The antioxidants in Okara Koji (OK), an okara (OC) fermented by Aspergillus oryzae, gamma-tocopherol, delta-tocopherol, genistin, daizein, genistein, and 3-hydroxyanthranilic acid were identified by HPLC. OK's extract with 80% methanol strongly inhibited linoleate peroxidation, much more than other OK's extracts with hexane or hot water. The methanol extract accelerated 12-hydroxyeicosatetraenoic acid formation in membrane lipids at 10(-3) concentration, but inhibited the formation at higher concentrations than 10(-3) ex vivo. To confirm the total effect of all components of OK on lipid peroxidation in vivo, rats fed food deficient in vitamin E were put on diets containing OK or OC with oxidized oil. In rats fed the OK diet, no effect of oxidized oil feeding on the body weight gain, of the TBA value in plasma, or of glutathione peroxidase activities of plasma and liver was observed. But in rats fed the OC diet, the effect of oxidized oil feeding was apparently observed on all of those values. These results suggested that OK would scavenge lipid peroxides in vivo.