A sulfonic acid (SO3H)-group-containing nonwoven fabric was prepared by radiation-induced graft polymerizaton of an epoxy-group-containing monomer, glycidyl methacrylate, onto a nonwoven fabric and subsequent sulfonation. The nonwoven fabric containing SO3H groups at different densities was equilibrated with lysozyme in a buffer (pH 6.0). The equilibrium binding capacity (EBC) of lysozyme-increased linearly with increasing SO3H group density. The SO3H-group-containing polymer chains extended from the pore surface due to mutual electrostatic repulsion and held lysozyme in multilayers. The maximum EBC was 0.7 kg/kg at a SO3H group density of 1.7 mol/kg.