A water-soluble polysaccharide (LZ-D-1) was isolated from Ganoderma lucidum fruiting bodies by DEAE-Sepharose Fast Flow and Sephacryl S-300 high-resolution chromatography. The polysaccharide had a molecular weight of 2.8 x 10(4) Da and was mainly composed of L-Fuc, D-Glc and D-Gal in a molar ratio of ca.1 : 1 : 5. LZ-D-1 had a sugar content of 98.7 % as measured using the phenol-sulfuric acid method. On the basis of methylation analysis, FT-IR spectrum and NMR experiments, the structure of the repeating unit of the polysaccharide was established. LZ-D-1 had a backbone composed of 1,6-disubstituted alpha-D-galactopyranosyl, 1,2,6-trisubstituted alpha-D-galactopyranosyl and had a branch composed of fucose attached to O-2 of a 1,2,6-trisubstituted alpha-D-galactopyranosyl residue. At the same time, the polysaccharide contained some 1,6-disubstituted glucopyranosyl residues and 1,3-disubstituted glucopyranosyl residues. An immunostimulating experiment in vitro indicated that LZ-D-1 stimulated proliferation of mouse spleen lymphocytes.