The endoxylanase (1,4-beta-D-xylan xylanohydrolase, EC 3.2.1.8) from the culture filtrate of a mushroom, Termitomyces clypeatus, has been purified 93-fold by ammonium sulphate precipitation, ion-exchange chromatography (DEAE-Sephadex) and gel permeation chromatography (Bio-Gel P-200). The enzyme preparation gave a single protein band on disc gel electrophoresis at pH 9.5, and has a molecular weight of approx. 90 000. It also acts on amylopectin, arabinoxyland and arabinogalactan. The enzyme shows maximum activity on xylan (1,4-beta-linked D-xylopyranose units) at pH 55.5 and at 55 degrees C and is fairly stable between pH 3 and 10 and temperatures up to 60 degrees C. The Km is 4 mg of xylan/ml. Hg2+ is the most potent inhibitor, whereas Fe2+, Ag+, iodoacetate and phosphate moderately inhibit the enzyme activity.