A water-soluble polysaccharide was isolated from Area subcrenata Lischke (named ASLP) by hot-water extraction, anion-exchange, and gel-permeation chromatography. The average molecular weight of ASLP was estimated to be 3500 Da. The structural characterization of ASLP was performed by sugar composition analysis, methylation analysis, and partial acidic hydrolysis. Further analysis of ASLP was carried out by UV, FT-IR and NMR spectroscopies (ID, COSY, and HSQC, respectively). Our data suggests that ASLP is an alpha-(1 -> 4)-D-glucan, with an alpha-(1 -> 6)-D-glucan at the C-6 position every fourth residue along the main chain. The branch chain has three glucose residues. The possible structure, determined on the basis of structural analyses results, was also determined. Preliminary in vitro tests revealed that ASLP can stimulate mouse spleen lymphocyte proliferation and its branches are extremely important for its immunological activity.