Two extracellular xylanases produced by the thermotolerant fungus Aspergillus caespitosus grown in sugar cane bagasse were purified and characterized. Estimated molecular masses were 26.3 and 27 kDa (xyl I); 7.7 and 17.7 kDa (xyl II) for gel filtration and SDS-PAGE, respectively. Optimal temperature for both xylanases was 50-55 degrees C. Optimal pH was 6.5-7.0 for xyl I, and 5.5-6.5 for xyl II. The thermostability (T-half) at 55 degrees C was 27.3 min (xyl I) and > 90 min (xyl II). Xylanase activity was inhibited by several ions. beta-mercaptoethanol activated 59 and 102% xyl I and xyl II activities, respectively. These enzymes preferentially hydrolyzed birchwood xylan, and the K-m and V-max values were 2.5 mg/ml and 1679 U/mg protein (xyl I), and 3.9 mg/ml and 113 U/mg protein (xyl II). The action of both xylanases mainly that of xyl II, on kraft pulp reduced kappa number and increased pulp viscosity. (c) 2004 Elsevier Ltd. All rights reserved.