Protein transport and separation properties of a novel, surface-modified poly (vinyl alcohol) (PVA) gel membrane were studied. Permeation experiments with bovine serum albumin (BSA) and lysozyme were performed in a stirred diffusion cell through unmodified and surface-modified PVA gels. The PVA gels were cross-linked with glutaraldehyde and surface modified by interfacial polymerization with toluene 2,4-diisocyanate (TDI) in hexane. The TDI concentration and interfacial contact time were varied to find the optimum selectively and permeability conditions. It was found that a reaction time of 14 min at a 1% TDI concentration resulted in the highest selectivity, 105, for the flux of lysozyme relative to that for BSA. Compared to an unmodified gel, the surface-modified membrane exhibited a 4.5-fold enhancement in selectivity for lysozyme versus BSA while the lysozyme flux retained 76% of its value. The surface modification was useful in increasing membrane selectivity while still maintaining good permeability. A modified version of the multiple-membrane technique was used to determine liquid-phase resistances, so the intrinsic membrane resistance could be evaluated. These boundary layer resistances were found to be negligible.