A water-soluble polysaccharide with a molar weight of 4.3x10(5)Da, termed as HBP was isolated from the fruit bodies of an edible mushroom, Sarcodon aspratus (Berk.) S. Ito. Composition, methylation analysis, Fourier transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy experiments were conducted to elucidate its structure. The results indicated that HBP is a glucan featuring a backbone of (1-->6)-linked-beta-d-glucopyranosyl residues, which occasionally branched at O-3 position on along the backbone and substituted by the side chains that consisting of (1-->3)-linked-beta-d-glucopyranosyl, (1-->4)-linked-beta-d-glucopyranosyl and non-reducing end beta-d-glucopyranosyl residues. Immunological activity evaluation by H(3)-thymidine incorporation method revealed that HBP can significantly stimulate the proliferation of the cultured mice spleen lymphocyte in a dose-dependent manner, thus, HBP is a promising potential immunomodulator that can be used as healthcare food or medicine against pathogens and tumors.