Bacillus stear other mophilus T-6 produces an extracellular thermostable xylanase that can bleach paper pulp optimally at 60 degrees C and pH 9.0. We developed an efficient method for purifying the enzyme from the cell broth by using successive steps of batch adsorption on the cellulosic cation exchanger SE-52. The optimal pH values for adsorption and elution are 5.5 and 9 respectively. The conductivity of the cell broth should remain below 7 mS/cm(2) and the suitable temperature range for adsorption is 15-60 degrees C. The adsorption parameters are : maximum capacity, 118 mg enzyme/g adsorbent; dissociation constant, 0.6 mg/ml; partition coefficient 0.988, On the basis of these results, a large-scale (12 000 1) purification process was carried out resulting in a 55% final yield and over 95% pure enzyme.