Two endoxylanases (EC 3.2.1.8), xyl1 and xyl2, were purified by subsequent anion-exchange, size-exclusion, and cation-exchange chromatography from a commercial enzyme preparation derived from the thermophilic fungus Humicola insolens. The homogeneous proteins had molecular masses of 6 and 21 kDa (SDS polyacrylamide gel electrophoresis) and isoelectric points of 9.0 and 7.7, respectively. The low molecular weight of xyl1 was confirmed by mass spectrometry. Both enzymes had similar pH and temperature optima (pH 6-6.5 and 55-60 degrees C) but their stability at various pH and temperatures differed. The molar activity towards xylans from beech, birch, larch, and arabinoxylans from wheat was higher for xyl2. Both xylanases had remarkably lower molar activities toward the isolated insoluble fractions of these xylans or toward the essentially insoluble beech xylan, but the decrease was relatively less pronounced with xyl2. These findings might be explained by differences in specific adsorption : xyl2 adsorbed strongly onto insoluble beech xylan while the affinity of xyl1 was much lower. In contract to xyl1, xyl2 was markedly inhibited by a number of metal ions. The reaction products formed during hydrolysis of different xylans and the end products (xylobiose, xylotriose, minor amounts of monomeric xylose, and substituted [(4-o-methyl)glucurono]arabino-xylooligomers) were equal for both enzymes, but their relative proportions differed slightly.