The use of the chain transfer agent, 2-mercaptoethanol, and ethanol cleaning was successful in producing hydrophilic membranes with higher permeabilities than the unmodified poly(ether sulfone) (PES) membrane. Permeabilities of dip-modified membranes were three to five times higher than with ethanol cleaning alone. Membrane permeability increased by 50% for dip-modified membranes in the presence of a low 2-mercaptoethanol concentration and by 20-200% when high 2-mercaptoethanol concentrations were used. From the lower degree of grafting (DG) achieved. it was inferred that the graft chain density and chain length decreased with increasing concentration of chain transfer agent. However, the observed rejection was severely reduced indicating that dip-modification caused considerable pore enlargement. This was not previously observed because non-grafted homopolymer may have been formed which effectively blocked the pores. It is speculated that ethanol cleaning removed the homopolymer because it wets and possibly swells the membrane pore structure. This indicated that it was not possible to dip-modify 50 kDa PES UF membranes using 300 nm lamps and a benzene filter without causing significant chance to the pore structure. In addition. the combination of irradiation and N-vinyl-2-pyrrolidinone (NVP) caused a more severe loss of observed rejection than irradiation alone. The molecular weight cut-off (MWCO) of membranes irradiated and/or dip-modified with or without 2-mercaptoethanol was estimated to be between 50 and 100 kDa using a calibration curve for observed rejection versus membrane MWCO. The high hydrophilicity, high permeability, and low fouling character of membranes modified with 2-mercaptoethanol and cleaned with ethanol make them desirable for the filtration of larger proteins (>500 kDa) or for applications with smaller proteins in which high protein transmission is required.