The electrophoretic mobilities of hemoglobin and lysozyme were measured through polycarbonate track-etched membranes of different pore sizes. Together with the zeta potential of the protein-fouled membranes, and measurements of the free-solution mobilities, protein sizes, and membrane pope sizes, the theory of Ennis et al. was tested, The presence of the membrane offered little hindrance to protein transfer when the membrane pore size was large in comparison with the protein size and the thickness of the electrical double layers. Under some solution conditions, protein agglomeration was significant and the interactions between the larger particles, and the membrane pore walls caused a more pronounced reduction in the protein mobility from its free-solution value. Good agreement with the theoretical model was found only for cases where the solution remained as a monodispersed suspension of protein monomers.