Endoxylanases from the thermophilic fungus, Thermomyces lanuginosus ATCC 44008 (cellulase free wild and mutant strains), were purified to homogeneity by anion-exchange and molecular-sieve chromatographic methods. The purified enzymes were monomers with molecular masses of 22 kDa (wild type) and 24 kDa (mutant), estimated by SDS-PAGE and gel filtration. As glycoproteins, the purified enzymes had 0.74% (wild type) and 11.8% (mutant) carbohydrate contents, and pI values of 5.8 and 6, respectively. The optimal pH and temperature values of wild type xylanase were determined to be pH 7 and 60 degreesC, whereas pH 6.7 and 70 degreesC, were optimal for the purified mutant enzyme (K-m and V-max values of 3.7 mg ml(-1) and 670 mumol min(-1) xylose compared to the kinetic values of the purified wild type xylanase 5.1 mg ml(-1) and 385 mumol min(-1) xylose). Inhibition studies suggested the possible involvement of histidine, tryptophan residues and carboxylic groups in the binding or catalysis.