Experimental data are obtained for bovine serum albumin transport through an asymmetric poly(ether sulfone) ultrafiltration membrane over a wide range of salt concentrations. The actual membrane sieving coefficients are determined from the measured filtrate concentrations using a stagnant film model that accounts for bulk mass transfer effects in the stirred device. The relative contributions of the diffusive, convective, and "electrophoretic" transport to the overall protein flux were then evaluated using a theoretical model for transport in charged cylindrical pores. The electrophoretic transport arises from the streaming potential that is generated by the convective solvent flow through the membrane, with this electrophoretic effect being comparable to convection at very low salt concentrations. The intrinsic membrane transport parameters are then compared with available hydrodynamic predictions. The dominant effect of the electrical interactions on protein transport is through the alteration of the partition coefficient, although the hydrodynamic drag factors also appear to be a function of salt concentration.