N, N–dimethyl–γ–aminobutyric acid (DMGABA) as a carboxybetaine was introduced into the polymer chain grafted onto the pore surface of a polyethylene-made porous hollow-fiber membrane. The DMGABA–immobilized porous hollow-fiber membrane (DMGABA fiber) was prepared by the radiation–induced graft polymerization of glycidyl methacrylate (GMA) and the subsequent addition of DMGABA to the epoxy group of the grafted poly–GMA chain. The dose of the electron beam, the degree of GMA grafting (dg), and the molar conversion of the epoxy group into the DMGABA group were optimized to minimize the amount of protein adsorbed. As a result, a dose of 200 kGy, a dg of 140%, and a molar conversion above 3% were selected from the viewpoints of the protein binding capacity of the DMGABA fiber. The binding capacity of the DMGABA fiber for lysozyme in carbonate buffer (pH 9.0) was reduced to less than 10% that of the original polyethylene membrane. The lysozyme binding capacity of the DMGABA fiber in Britton–Robinson universal buffer (pH 7.0) was 17 ng per cm2 of the pore surface.