The protein-bound polysaccharide of Coriolus versicolor QUEL (PS-K) has been found to express antioxidant activity as an "ion-radical scavenger" in diamine oxidation reactions. The mode of this expression was examined to determine whether the drug functioned as a simple radical scavenger or mimicked the action of superoxide dismutase (SOD). The latter was confirmed in both enzymatic and nonenzymatic superoxide anion radical (O2-.) producing systems in vitro. The SOD mimetic activity of PS-K was demonstrated by quantitative analysis of hydrogen peroxide as the end product of O2-., its formation being assisted catalytically by SOD or PS-K. Analysis by electron spin resonance also confirmed the SOD mimetic activity of PS-K in a xanthine-xanthine oxidase reaction. Relative SOD activity with PS-K was approximately 1/8,000 in a KO2-O2-.-producing system. The SOD mimetic activity of PS-K resisted treatment by 0.7N HCl, 0.7N NaOH, boiling for 30 minutes in a double water bath, and digestion by pronase. Fractionation according to differences in molecular mass caused no significant increase in relative SOD activity within a certain range of molecular mass, indicating that there is no definite molecule expressing SOD mimetic activity. Tumor-bearing rats and human patients with digestive tract cancer who suffered from oxidative stress were relieved by a single intraperitoneal administration of PS-K or a 1-day peroral prescription.