An endoxylanase (1,4-beta-D-xylan xylanohydrolase, EC 3.2.1.8) was isolated from the culture filtrate of one isolated strain of Trichoderma longibrachiatum CS-185 grown on oat spelt xylan. The enzyme was purified to homogeneity by sequential ammonium sulfate fractionation and ion-exchange and gel filtration chromatographies. The apparent purity was demonstrated by SDS-PAGE and a zymogram analysis subsequent to a native PAGE. The molecular mass of the xylanase was 37.7 kDa. The optimal pH and temperature for activity were 5.0-6.0 and 45 degrees C, respectively. The enzyme with oat spelt xylan as substrate had a K-m of 10.14 mg xylan ml(-1) and a V-max of 4,025 U mg(-1) protein. The enzyme was active on both oat spelt and birch xylans. The products observed on TLC after a 23-h hydrolysis of the enzyme oil oat spelt xylan were xylobiose, xylotetraose, and xylooligosaccharides with greater chain length.