The antioxidant effects of Hypsizigus marmoreus, one of the most popular Japanese edible mushrooms, were investigated by the use of the augmentation effect of antioxidant activity (AOA) in the mice plasma. An aqueous extract of the mushroom fruit-body was found to have a slight trap activity for peroxyl and alkoxyl radicals. On the other hand, the blood plasma of mice fed with a fodder containing 5-10% of the dried powder of the mushroom extract augmented significantly AOA for alkoxyl radicals. It was suggested from analysis of the plasma by the HPLC post column AOA method that the increase of AOA in the mice plasma was caused by the induction of high molecular weight fractions having AOA produced by feeding Hypsizigus marmoreus. The amount of lipoperoxide in the mice plasma as values of thiobarbituric acid reactive substances showed a tendency to be lowered by intake of Hypsizigus marmoreus. These results suggest that oral administration of the fruit-body of Hypsizigus marmoreus can induce an antioxidant effect in the mice plasma.