The study of the electrostatic interactions in the ultrafiltration of single model proteins is an important topic for the biotechnology industry interested in the fractionation of protein mixtures. In this paper, the dynamics of the ultrafiltration of P-lactoglobulin and lysozyme through a 300 kDa tubular ceramic membrane was investigated. The time evolution of permeate flux and protein transmission were determined at a temperature of 30 degrees C, a cross-flow velocity of 3.5 m/s and a transmembrane pressure of 100 kPa. The influence of pH was monitored in the 3-9 interval for P-lactoglobulin and in the 5-12 interval for lysozyme. The effect of ionic strength was studied by increasing the addition of NaCl up to 15 mM. The curves of permeate flux were discussed according to the resistances in series model. Since P-lactoglobulin and lysozyme have acid and alkaline isoelectric points, respectively, the results showed different responses with respect to the electrostatic environment. (C) 2007 Elsevier B.V.. All rights reserved.