A novel water-soluble polysaccharide (POPS-1) was obtained from the fruiting bodies of Pleurotus ostreatus by hot water extraction, ethanol precipitation, and fractionated by DEAE-cellulose ion exchange chromatography and sepharose CL-6B gel filtration chromatography using an ATKA explore 100 purifier. The structure characterization and antitumor activity of the POPS-I were evaluated in this paper. According to GC analysis, HPGPC, FT-IR, partial acid hydrolysis, periodate oxidation and Smith degradation, methylation and GC-MS analysis, the results indicate POPS-1 (M(w) = 31 kDa) was composed of Man; Gal; Glc with a molar ratio of 1:2.1:7.9, it had a backbone of beta-(1 -> 3)-linked glucose residues, which occasionally branches at O-6. The branches were composed of (1 -> 3)-linked Glc, (1 -> 4)-linked Gal, (1 -> 4)-linked Man, and terminated with Glc and Gal residues. Cytotoxicity assay showed POPS-1 presented significantly higher antitumor activity against Hela tumor cell in vitro, in a dose-dependent manner, and exhibited significantly lower cytotoxicity, to human embryo kidney 293T cells than Hela tumor cells compared with 5-Fu. The results suggest POPS-1 may be considered as a potential candidate for developing a novel low toxicity antitumor agent. (C) 2008 Elsevier Ltd. All rights reserved.