An alkali-insoluble core material was isolated from the fruit bodies of Lentinus edodes after exhaustive extraction with 24% NaOH at 5 degrees C. This material consists mainly of glucan which is closely associated with chitin. Methylation analysis has shown that the glucan part of the core material (skeletal glucan) has a highly branched structure with 1,6 and 1,3 linkages in a molar ratio of 2 : 1. Stepwise enzymatic hydrolysis with basidiomycete sp. QM 806 beta-1,3-glucanase has indicated the heterogeneity of the skeletal glucan. The outer part of the skeletal glucan seems to be composed mainly of beta-1,3 and beta-1,6 glucoside linkages and has a close structural similarity to lentinan, a water-soluble beta-glucan from L. edodes. The middle part of the skeletal glucan appears to be composed mainly of beta-1,6 glucoside linkages. The innermost part of the skeletal glucan is a highly branched glucan with beta-1,6 and beta-1,3 linkages. Probably, it is associated with chitin and a small amount of amino acid polymer.