The main component of a water extract of Boletus erythropus fruiting bodies is a M(r) 10(6) glucan. The use of classical structural analysis and HMQC (heteronuclear multiple quantum coherence) NMR experiments indicates a (1-->3) linked beta-D-glucan structure with a single glucose residue attached to O-6 of the main chain and a branching frequency of 1/3.