1. The culture filtrate of the fungus Coniophora cerebella grown on poplar 4-O-methylglucuronoxylan as carbon source and enzyme inducer contained an enzyme system that degraded the polysaccharide to xylose, acidic and neutral oligosaccharides and an enzyme-resistant polymer. Free uronic acid was not produced. 2. Cold ethanol fractionation of the culture filtrate yielded two active fractions, one of which had only xylanase (EC 3.2.1.8) and the other both xylanase and xylosidase (EC 3.2.1.37) activities. Further fractionation on DEAE-cellulose resolved the xylanase and xylosidase activities. 3. The xylanase degraded poplar 4-O-methylglucuronoxylan in an essentially random manner, producing oligosaccharides, but some xylose residues in the vicinity of uronic acid side groups were protected from hydrolysis, preventing a truly random attack. The xylosidase attacked the polysaccharide very slowly, releasing xylose, but the oligosaccharides produced by the action of the xylanase were much more susceptible to hydrolysis by the xylosidase. 4. The products of xylanase action were separated into neutral and acidic fractions. The neutral oligosaccharides were separated by chromatography on charcoal–Celite, and the major products were characterized as xylobiose, xylotriose, xylotetraose and xylopentaose. Some of the acidic sugars were branched, having the uronic acid residue attached to a xylose residue other than the terminal non-reducing one. 5. Gel filtration of various xylanase fractions gave values for the molecular weight of the enzyme from 34000 to 38000.
