Although many current applications of membrane filtration involve the removal, concentration, or exchange of electrolytes, there is still no fundamental understanding of the effects of ionic composition and concentration on the performance of these filtration processes. We have obtained experimental data for the effect of ionic species on the hydraulic permeability of bovine serum albumin deposits formed during filtration. The permeability of the protein deposits decreased with increasing solution ionic strength at pH both above and below the protein isoelectric point but was relatively independent of ionic strength at the isoelectric point. The permeability of the BSA deposits at neutral pH was markedly smaller when measured with solutions of divalent cations than with solutions of monovalent cations but was unaffected by the valence of the anions. This dependence on ionic strength and ion valence was consistent with a simple model in which the protein layer permeability is determined by the balance between the compressive pressure associated with the filtration and the electrostatic repulsion between the negatively charged proteins in the deposit. These results were then combined with available data for the albumin diffusivity and osmotic pressure to examine the effects of solution ionic strength on the flux during protein filtration.