The biobleaching effect of cellulase-free thermophilic xylanase preparations from Streptomyces thermoviolaceus on birchwood kraft pulp was determined. Enzyme pretreatment was found to be most affective at high pulp consistencies and resulted in the hydrolysis of hemicellulose and the release of chromophoric material from the pulp. Removal of residual lignin was indicated by a reduction in Kappa number. The application of cyclic enzyme treatments coupled with intermediate alkaline extraction stages increased the biobleaching effect however, more then two cycles resulted in a decrease in the physical strength of the paper. The enzyme treatment process was optimized and found to be most effective at 65 degrees C and pH 6.0, with the crude enzyme preparation blended to a pulp consistency of 5%. A 3 h enzyme pretreatment of pulp followed by single-stage peroxide bleaching yielded pulp with a brightness of >70 ISO units for the production of paper handsheets in which the physical strength of the fibers had been presented.