Protein deposits formed during membrane filtration can dramatically reduce the effectiveness of these devices. We have obtained data for the hydraulic permeability of deposits formed by the filtration of albumin, lysozyme, ribonuclease A, hemoglobin, and immunoglobulins. The steady-state permeability for each of the protein deposits was minimum at the protein isoelectric pH and decreased with increasing salt concentration at pH both above and below the isoelectric point. The transient response of these protein deposits to an increase in solution ionic strength consisted of an instantaneous jump in flux followed by a relatively slow flux decline. This complex behavior was consistent with the effects of the solution environment on the protein packing density within the deposit (which is determined by the balance between the intermolecular electrostatic repulsion between protein molecules and the compressive pressure associated with filtration) and on the magnitude of the electro-osmotic counterflow associated with the streaming potential that is generated across the deposit. These results provide important insights into the physical phenomena governing the structure and hydraulic permeability of the protein deposits formed during membrane filtration.