Recent experiments have demonstrated that PEGylated proteins, produced by covalent attachment of a polyethylene glycol chain to a native protein, can be purified using negatively charged ultrafiltration membranes. However, the nature of the electrostatic interactions governing the transmission of these PEG-protein complexes during ultrafiltration has not been examined. Experimental data were obtained for the transmission of PEGylated a-lactalbumin through both unmodified and negatively charged composite regenerated cellulose membranes over a range of solution ionic strength. The effect of the polyethylene glycol on the protein was also studied by measuring the electrophoretic mobility of both PEGylated and acetylated versions of the a-lactalbumin. The attachment of the PEG altered the protein charge, the effective size, and the electrostatic potential surrounding the protein. The PEGylated a-lactalbumin was strongly retained by the negatively charged membrane due to the significant electrostatic interactions in this system. The data were consistent with a simple model of the PEGylated protein as an effective hard sphere with the surface charge shifted to the outer edge of the PEG by accounting for the alteration in the electrostatic potential associated with the reduced electrolyte concentration within the PEG layer. These results provide important insights into the role of electrostatic interactions on the retention of PEGylated proteins during ultrafiltration. (C) 2010 Elsevier B.V. All rights reserved.