The water-soluble polysaccharide (POP), with a molecular mass of 2.4 x 10(4) Da, was obtained from the fruiting body of Pleurotus ostreatus. Structure features of the purified polysaccharide were investigated by a combination of chemical and instrumental analysis, such as methylation analysis, Smith degradation, GC-MS, (13)C and (1)H NMR and FTIR. The results indicated that the backbone of POP was composed of (1 -> 6)-linked-alpha-D-galactopyranosyl and (1 -> 2,6)-linked-alpha-D-galactopyranosyI residues, which were terminated with a single terminal (1 ->)-beta-D-glucose residue at the O-2 position of galactosyl along the main chain in the ratio of 1:1:1. Preliminary tests in vitro showed POP is capable of enhancing concanavalin A (ConA)- or lipopolysaccharide (LPS)-induced lymphocyte proliferation, which suggested that POP Could be a potential immunostimulating agent for use in functional foods or medicine against both pathogens and cancer. (C) 2008 Elsevier Ltd. All rights reserved.