Defatted soybean-ontjom (D-ontjom, defatted soybean fermented with Neurospora intermedia) was prepared to utilize defatted soybean (DSB) as a food source. The antioxidant activity of hydrophilic components of D-ontjom and the mechanism were investigated. The 1,1-diphenyl-2-picryl-hydrazyl-radical scavenging activities of three extracts from D-ontjom, hexane, methanol and water extracts, were all stronger than those from DSB. Among them, the water extract from D-ontjom showed the strongest activity. I prepared an 80% methanol soluble fraction (hydrophilic fraction) from the water extract, which showed much stronger DPPH-radical scavenging activity. This hydrophilic fraction strongly suppressed the generation of superoxide anion and glutathione-autoxidation. To confirm the antioxidant activity of the hydrophilic fraction in vivo, rats were fed the fraction prepared from D-ontjom or DSB. The serum and liver thiobarbituric acid reactive substance (TBARS) values, hepatic superoxide dismutase (SOD, EC 1.15.1.1) activity and serum glutathione peroxidase (GSH-PX, EC 1.11.1.9) activity in rats fed the D-ontjom diet were significantly lower compared with those in rats fed the DSB diet, while their serum levels of α-tocopherol and glutathione were remarkably higher than those of rats fed the DSB diet. These results suggest that some hydrophilic components of D-ontjom have a high antioxidant activity in vivo, perhaps due to their radical scavenging activity.