Cordyceps sinensis, a well-known traditional Chinese medicine, possesses activities in anti-tumour, anti-oxidation and stimulating the immune system; however, the identity of active component(s) is not determined. By using anti-oxidation activity-guided fractionation, a polysaccharide of molecular weight ∼210 kDa was isolated from cultured Cordyceps mycelia by ion-exchange and sizing chromatography. The isolated polysaccharide, having strong anti-oxidation activity, contains glucose, mannose and galactose in a ratio of 1:0.6:0.75. The pre-treatment of isolated polysaccharide on the cultured rat pheochromocytoma PC12 cells shows strong protective effect against hydrogen peroxide (H2O2)-induced insult. Treatment of the cells with the isolated polysaccharide at 100 μg/ml prior to H 2O2 exposure significantly elevated the survival of PC12 cells in culture by over 60%. In parallel, the H2O 2-induced production of malondialdehyde in cultured cells was markedly reduced by the polysaccharide treatment. Moreover, the pre-treatment of the isolated polysaccharide significantly attenuated the changes of glutathione peroxidase and superoxide dismutase activities in H 2O2-treated cells in a dose-dependent manner. This is the first report in identifying a polysaccharide from Cordyceps, which protects against the free radical-induced neuronal cell toxicity. © 2003 Elsevier Inc. All rights reserved.