Endopolysaccharides represented by glucans, galactans, and glycoproteins were isolated from the fruiting bodies of the sulfur shelf (Laetiporus sulphureus (Bull.:Fr.) Murr), which were obtained by the natural-plantation method. The study of the major 56-kDa polysaccharide, laetiporan A, the content of which accounted for 0.28% of the fruiting body weight, showed that it is a beta-1,3-glucan containing mannose, galactose, fucose, xylose, and rhamnose residues at position C-6. An antioxidant effect of laetiporan A was discovered.