Two kinds of ionizable groups, namely, diethylamino and sulfonic acid groups, were introduced to a polymer brush by reaction of the epoxy groups of poly(glycidyl methacrylate) grafted onto st porous hollow-fiber membrane with diethylamine and sodium sulfite, respectively. Aminoacylase solution was permeated through the pores under a constant permeation pressure, and the changes in both the protein concentration of the effluent and the permeation pressure were continuously monitored. Aminoacylase was bound in multilayers to the polymer brush based on an ion-exchange interaction. The diethylamino-group-containing polymer brush extended with the capture of the protein, whereas the sulfonic acid-group-containing polymer blush shrank with the progression of protein binding.