Whenmycelia of Streptomyces sp. No. 3137 were cultivated in a mediumcontaining methyl fixyloside, xylanases (EC 3.2. 1.8) were inductively produced into the medium. Three types of enzyme from the culture filtrate have been purified by ultra filtration with DIAFLOUM-10, chromatography on DEAE-Sephadex A-25, gel filtration on Bio Gel P-100, and isoelectric focusing with Servalyt 6-8 or 9-ll. The three purified enzymes, tentatively named X-I, X-II-A, and X-II-B, were homogeneous by polyacrylamide gel electrophoresis at pH 4.3. The molecular weight of X-I was about 50,000 by SDS-polyacrylamide gel electrophoresis or gel filtration on Bio Gel P-100. The molecular weight of X-II-A and X-II-B were both approximately 25,000 by SDS-polyacrylamide gel electrophoresis and that of X-II-B was 25,680 by the sedimentation-equilibrium method. X-I had an isoelectric point at 7.10, and X-II-A and X-II-B had different isoelectric points, 10.06 and 10.26, respectively. The three enzymes were optimally active at 60 -65°C and stable to 55°C. The optimal pH ofX-I, X-II-A, and X-II-B were pH 5.5-6.5, 5.0-6.0, and 5.0-6.0, respectively.The ranges of two X-IFs pH stability (pH 1.5-ll.5) were wider than that of X-I's (pH 3.0-10.5). These purified preparations hydrolyzed xylotriose, xylotetraose, and xylan but not xylobiose, cellobiose, maltose, carboxymethyl cellulose, or soluble starch. Their actions were inhibited by Hg2+ and Fe3+ ions, sodium dodecyl sulfate, and A^bromosuccinimide.