A xylanase purified from the thermophilic fungus Thermomyces lanuginosus CBS 288.54 was characterized and its potential application in wheat straw pulp biobleaching was evaluated. Xylanase was purified 33.6-fold to homogeneity with a recovery yield of 21.5%. It appeared as a single protein band on SDS-PAGE gel with a molecular mass of approx. 26.2 kDa. The purified xylanase had a neutral optimum pH ranging from pH 7.0 to pH 7.5, and it was also stable over pH 6.5-10.0. The optimal temperature of the xylanase was 70-75 degrees C and it was stable up to 65 degrees C. The purified xylanase was found to be not glycosylated. The xylanase was highly specific towards xylan, but did not exhibit other enzyme activity. Apparent K. values of the xylanase for birchwood, beechwood, soluble oat-spelt and insoluble oat-spelt xylans were 4.0, 4.71 2.0 and 23.4 mg ml(-1), respectively. The potential application of the xylanase was further evaluated in biobleaching of wheat straw pulp. The brightness of bleached pulps from the xylanase pretreated wheat straw pulp was 1.8-7.79% ISO higher than that of the control, and showed slightly lower tensile index and breaking length than the control. Although chlorine consumption was reduced by 28.3% during bleaching, the xylanase pretreated pulp (15 U g(-1) pulp) still maintained its brightness at the control level. Besides, pretreatment of pulp with the xylanase was also effective at an alkaline pH as high as pH 10.0. (c) 2004 Published by Elsevier Ltd.