Melanocarpus albomyces, a thermophilic fungus isolated from compost by enrichment culture in a liquid medium containing sugarcane bagasse, produced cellulase-free xylanase in culture medium. The fungus was unusual in that xylanase activity was inducible not only by hemicellulosic material but also by the monomeric pentosan unit of xylan but not by glucose. Concentration of bagasse-grown culture filtrate protein followed by size-exclusion and anion-exchange chromatography separated four xylanase activities. Under identical conditions of protein purification, xylanase I was absent in the xylose-grown culture filtrate. Two xylanase activities, a minor xylanase IA and a major xylanase ILIA, were purified to apparent homogeneity from bagasse-grown cultures. Both xylanases were specific for beta-1,4 xylose-rich polymer, optimally active, respectively, at pH 6.6 and 5.6, and at 65 degrees C. The xylanases were stable between pH 5 to 10 at 50 degrees C for 24 h. Xylanases released xylobiose, xylotriose and higher oligomers from xylans from different sources. Xylanase IA had a M-r of 38 kDa and contained 7\% carbohydrate whereas xylanase IIIA had a M-r of 24 kDa and no detectable carbohydrate. The K-m for larchwood xylan (mg ml(-1)) and V-max (mu mol xylose min(-1) mg(-1) protein) of xylanase IA were 0.33 and 311, and of xylanase IIIA 1.69 and 500, respectively. Xylanases IA, II and ILIA showed no synergism in the hydrolysis of larchwood glucuronoxylan or oat spelt and sugarcane bagasse arabinoxylans. They had different reactivity on untreated and delignified bagasse. The xylanases were mole reactive than cellulase on delignified bagasse. Simultaneous treatment of delignified bagasse by xylanase and cellulase released more sugar than individual enzyme treatments. By contrast, the primary cell walls of a plant, particularly from the legion of elongation, were more susceptible to the action of cellulase than xylanase. The effects of xylanase and cellulase on pl;int cell walls were consistent with the view that hemicellulose surrounds cellulose in plant cell walls.