Electrical properties for a range of virgin and protein-fouled polycarbonate track-etched (PCTE) membranes have been determined by streaming-potential measurements at pH 4-7 and 10(-3) M KCl at 25 degrees C. The apparent zeta potentials generally increased with increasing pore size above pH 5 and decreased with pore size below pH 5, For membranes with smaller pores (PCTE 0.01, 0.03 and 0.05 mu m) the zeta potentials became constant (the surface is fully charged) at about -6 mV at pH greater than or equal to 5, whereas those of larger pores (PCTE 0.1 and 0.2 mu m) showed constant zeta potentials of ca. -10 to -13 mV at pH > 6. In contrast, the protein-fouled membranes showed very similar apparent zeta potential vs. pH profiles for all pore sizes. This suggests that the measured zeta potentials are determined by the protein properties. The loss of strong pore-size dependence on zeta potentials upon protein fouling of membranes implies that membrane zeta potentials could be controlled by the conditions at pore entry, This ’model’ requires protein deposition around the pore openings for all pore sizes; this was in agreement with electron microscope observations. While the isoelectric points (IEP) of the virgin membranes determined from the pH dependence of streaming-potential measurements were between pH 3.7-4.4 with a trend to higher IEP for the larger pore membrane, all protein-fouled membranes showed IEP values at pH 4.6-4.8, which is close to the IEP of the BSA protein used. The chemical properties of the PCTE membranes investigated had FTIR spectra similar to those of polycarbonate from bis(4-hydroxyphenyl)-2-propane, bisphenol A polycarbonate. No BSA peak was revealed in the FTIR spectra of the small-pore membranes fouled with protein, possibly due to the insufficient instrument detection limit. For the membranes with larger pores, however, the BSA peaks increased with increasing pore size. The depth profiling of the fouled membranes indicated a predominant BSA adsorption on the surface or near the pore entries rather than throughout the membrane thickness, confirming the zeta potential ’model’ and electron microscope observations.