Streptomyces thermoviolaceus extracellular xylanase preparations were used to treat birchwood Kraft pulp prior to chlorine bleaching. Xylanase concentrations in the range 5-100 IU g(-1) dry weight pulp released reducing sugars and chromophores that absorb at 280 nm. At enzyme doses greater than or equal to 10 IU, further removal of chromophoric material was not significant although reducing sugar yields probably from oligosaccharide hydrolysis continued to increase. Pulp Kappa number was, educed by enzyme treatment with and without subsequent alkali extraction, and the pulp brightness after CEDED bleaching at 4% chlorine charge was boosted without significant reductions in fiber strength or interfiber bonding. Scanning electron microscopy revealed marked disruption and separation of pulp fibers even at low (5 IU g(-1) dry weight pulp) xylanase noses, and a 30-35% saving in the chlorine charge required to obtain pulp brightness comparable to controls could be achieved. Comparative and standardized treatments of pulp with the commercial bleach-boosting enzyme preparations Cartazyme and Pulpzyme demonstrated that the crude S. thermoviolaceus xylanase preparation was at least as effective in enhancing brightness with concomitant preservation of paper strength properties; however, S. thermoviolaceus xylanase does have the advantage of being active and stable at 65 degrees C and, like Pulpzyme but not Cartazyme, at neutral to alkaline pH values appropriate for applications in paper pulp processing.