Xylanase (EC 3.2.1.8) was purified from a crude extracellular enzyme of the wood-rotting fungus Trametes hirsuta by a four-step procedure involving precipitation with ammonium sulphate, gel filtration on Bio-Gel P-10, column chromatography on DEAE-Sephadex A-50 and preparative electrophoresis on polyacrylamide gel. The isolated enzyme, electrophoretically homogeneous, was separated from beta-xylo-sylosidase and other hydrolytic enzymes studied. The analysis of the degradation products of water-soluble (4-0-methylglucurono)-D-xylan from willow by the purified xylanase showed it to be endoxylanase (beta-1,4-xylan xylanohydrolase).