The brown-rot fungus Serpula lacrymans MAFF 420003 was grown in a liquid culture medium containing 0.5% carboxymethyl cellulose (CMC) and 1% glucose as carbon sources. Although little extracellular cellulase was secreted, the fungus produced an oxidized quinone-type chelator, 2,5-dimethoxy-1,4-benzoquinone (2,5-DMBQ). The concentration of 2,5-DMBQ in the medium reached a maximum of 90 μmm after a month of cultivation. S. lacrymans could reduce 2,5-DMBQ to 2,5-dimethoxyhydroquinone (2,5-DMHQ), thus a biological Fenton reaction was adopted by the fungus. The changes in the molecular weight distribution of CMC and arabinogalactan were analyzed after the addition of 2,5-DMHQ and Fe 3+ . CMC was apparently depolymerized by the reaction, but the same reaction conditions showed no significant effect on arabinogalactan. These differences suggest the specificities of the biological Fenton reaction via 2,5-DMBQ toward soluble polysaccharides. In addition, the crystallinity index of α-cellulose did not decrease as a result of the reaction with 2,5-DMHQ and Fe 3+ . These results provide indirect evidence that S. lacrymans employs a biological Fenton reaction mediated by a quinone-type chelator, and preferentially degrades amorphous regions of cellulose rather than crystalline regions in the non-enzymatic cellulose degradation.