A new water-soluble intracellular polysaccharide named as PTP, with a molecular mass of 3.7 x 10(4) Da, was obtained from the mycelium of Polyporus albicans (Imaz.) Teng. Structure features of the purified polysaccharide were investigated by a combination of chemical and instrumental analysis. The results indicated that PTP consisted of a backbone composed of (1 -> 3)-linked-beta-D-mannopyranosyl, (1 -> 3,6)-linked- beta-D-mannopyranosyl and (1 -> 6)-linked-alpha-D-galactopyranosyl residues in the ratio of 3:1:1, and terminated with a single non-reducing terminal (1 ->)-beta-D-mannopyranosyl residues at the C-6 position of (1 -> 3,6)-linked-beta-D-mannopyranosyl, along the main chain. This is the first report describing the isolation and structure elucidation of a new intracellular polysaccharide produced from the mycelium of P. albicans (Imaz.) Teng. Preliminary tests in vitro showed PTP have potent stimulating effects on murine lymphocyte proliferation induced by concanavalin A or lipopolysaccharide and its branches are extremely important for the expression of the enhancement of the immunological activity. (C) 2007 Elsevier Ltd. All rights reserved.