The protein-bound polysaccharide of Coriolus versicolor QUEL (PS-K) expresses the mimetic activity of superoxide dismutase (SOD). Human cancer patients usually suffer from oxidative stress (OS). Examination was made to determine the capacity of this drug with SOD mimetic activity for relieving OS. Rats transplanted with Walker 256 fibrosarcoma showed OS on day 12. After confirming high levels of OS on day 13, PS-K50 mg/kg was intraperitoneally administered, and prompt decrease in O2-release from RBC was noted. The drug ceased to have any effect 24 hours following the first inoculation. Average OS in human cancer patients was found twice that in healthy persons. In human cancer patients perorally administered PS-K3.0 g/day, OS decreased to the normal level one day after the initial administration. Plasma lipid peroxide (LPO) in cancer patients treated with PS-K for 28 days increased and withdrawal of the drug led to decreased LPO.