The high molecular weight xylanase from Chainia (NCL 82.5.1) is extracellular, cellulase-free and stable at alkaline pH (pH 8.0) at 50 degrees C. The enzyme showed inhibition by N-bromosuccinimide(NBS) and by cysteine-specific reagents p-hydroxy mercuric-benzoate(PHMB) and N-ethyl maleimide(NEM) implying that tryptophan and cysteine are present at or near the active site of. the enzyme. The enzyme was reversibly inhibited by low concentrations (0.5 M) of guanidine hydrochloride (Gdn.HCl) indicative of the presence of a carboxylate group in the active site of the enzyme. Kinetics of inactivation of enzyme by Gdn.HCl revealed that the essential carboxylate residues are present at the substrate-binding region of the enzyme.