The possible role of hydrogen peroxide in brown-rot decay was investigated by studying the effects of pretreatment of spruce wood and microcrystalline Avicel cellulose with H2O2 and Fe2+ (Fenton's reagent) on the subsequent enzymatic hydrolysis of the substrates. A crude endoglucanase preparation from the brown-rot fungus Poria placenta, a purified endoglucanase from Trichoderma reesei and a commercial Trichoderma cellulase were used as enzymes. Avicel cellulose and spruce dust were depolymerized in the H2O2/Fe2+ treatment. Mainly hemicelluloses were lost in the treatment of spruce dust. The effect of the pretreatment on subsequent enzymatic hydrolysis was found to depend on the nature of the substrate and the enzyme preparation used. Pretreatment with H2O2/Fe2+ clearly increased the amount of enzymatic hydrolysis of spruce dust with both the endoglucanases and the commercial cellulase. In all cases the amount of hydrolysis was increased about threefold. The hydrolysis of Avicel with the endoglucanases was also enhanced, whereas the hydrolysis with the commercial cellulase was decreased.