Xylanase obtained by the submersed fermentation of Thermomyces lanuginosus, a strain classified in the German type culture collection under the number DSM 5826, was purified to homogeneity by a combination of chromatographic methods and characterized. The molecular mass of the enzyme was estimated by SDS-electrophoresis, gradient gel electrophoresis, or gel filtration at 25.5 kDa, 24.0 kDa, or 22.5 KDa, respectively. The isoelectric point of the enzyme was determined to be pH 4.1. It was found that the enzyme hydrolyzed xylan as an endoxylanase and had practically no cellulolytic or any other similar hydrolytic activity. It exhibited the highest activity at a pH around 7.0 and in the temperature range of 60-70 degrees C, It was found that the enzyme contains only one cysteine residue which was important for the xylanase activity, since dithiobis-2-nitrobenzoic acid, p-hydroxymercuribenzoic acid, and Hg2+ ions completely inhibited the enzyme. Mn2+, Fe2+, and beta-mercaptoethanol enhanced the xylanase activity; the best results were obtained by the combined action of Fe2+ ions and beta-mercaptoethanol. Xylanase was stable for 96 h at a pH between 5.0-9.0 and at temperatures up to 60 degrees C. Significant stabilization of the enzyme was achieved by the addition of glycerol, beta-mercaptoethanol, or polyethylene glycol.