An anion-exchange-group-containing porous membrane in a hollow-fiber form was prepared by radiation-induced graft polymerization of glycidyl methacrylate and subsequent conversion of the produced epoxy group into a diethylamino group. Ascorbic acid oxydase (AsOM) was bound to the ionizable polymer chains grafted onto the pore surface during the permeation of ascorbic acid (AsA) solution through the pores. After crosslinking of the enzymes with glutaraldehyde, AsOM was immobilized at a resulting density of 130 mg per gram of the fiber, which amounted to the degree of enzyme multilayer binding of 12. The substrate solution was forced to permeate across the AsOM-multilayered porous hollow-fiber membrane of 1 mm thickness at a constant permeation rate ranging from 30 to 150 ml/h. Irrespective of the permeation rate experimented, quantitative conversion of AsA into dehydroascorbic acid was observed because the substrate was transported to the enzyme of high activity immobilized in multilayers by convective flow through the pores.