The two main xylanases produced by Trichoderma reesei were purified to electrophoretic homogeneity by ion-exchange and gel chromatography. The enzymes had isoelectric points of 9.0 and 5.5 and their molecular masses were 20 and 19 kDa, respectively. The purified xylanases were most probably distinct gene products, as they were also serologically dissimilar and had different proteolytic cleavage patterns. The pI 9.0 xylanase tolerated higher pH values and temperatures than the pI 5.5 xylanase. Both enzymes clearly preferred polymeric substrates to xylo-oligosaccharides. The substrate preference for polymeric xylans decreased with decreasing substitution (decreasing solubility). The apparent Km values for different xylans varied from 3.0 to 6.8 mg ml-1 for the pI 9.0 xylanase and from 14.8 to 22.3 mg ml-1 for the pI 5.5 xylanase. The pI 9.0 xylanase also showed transxylosidase activity with xylotetraose and xyloheptaose as substrates.