Abstract The separation of a β-xylanase by ultrafiltration from a mixture of xylanases and cellulases produced by Trichoderma reesei was investigated. The effects of the sizes and the pH dependent charges of the proteins and the membranes on separation were studied. A stirred cell device with hydrophobic polysulphone membranes of different cut-off values and manufacturers was employed and there were distinct differences in the pore charges of the virgin membranes from different manufacturers. The flux reductions for the membranes were high and the permeate fluxes dropped to the same flux level in spite of the large differences in the initial water fluxes. β-Xylanase (isoelectric point 9·0) selectivity increased with increasing pH and high selectivity values were obtained with PM30 membranes at pH 7·0.