Several recent studies have demonstrated that zwitterionic ultrafiltration (UF) membranes show very low fouling behavior due to the highly hydrated structure of the zwitterions. The objective of this study was to examine the rate of protein transport through a series of semipermeable zwitterionic ultrafiltration membranes generated by chemical modification of a base cellulose membrane. The extent of membrane modification was evaluated by X-ray photoelectron spectroscopy with the effective membrane surface charge density evaluated using streaming potential measurements. Ultrafiltration experiments were performed with basic, neutral, and acidic proteins (cytochrome c, myoglobin, and alpha-lactalbumin). Protein transport through the zwitterionic membranes was a strong function of solution conditions due to the electrostatic interactions between the charged proteins and the zwitterionic ligands. The protein sieving coefficient was well correlated with the product of the surface charge densities of the protein and membrane. There was no evidence of membrane fouling, even under conditions where the protein and membrane had opposite polarity. The results demonstrate the potential of using zwitterionic ligands to generate high performance ultrafiltration membranes for bioprocessing applications. (C) 2011 Elsevier B.V. All rights reserved.