Fractionation of bovine beta-lactalbumin (ALA) from 1s-lactoglobulin (BLG) in milk serum permeate (NiSP) using tangential flow ultrafiltration was examined in this work. By placing a positive charge on a 300 kDa regenerated cellulose ultrafiltration membrane, it was possible to increase the selectivity for fractionating ALA and BLG by 180% compared to an uncharged membrane. Thus, like-sized proteins that differed only somewhat in isoelectric point, and that were about 15-20 times smaller than the membrane molecular weight cutoff, were fractionated using charged ultrafiltration membranes. Electrostatic repulsion was solely responsible for the improved selectivity. Protein sieving coefficients were explained by the net charge on the protein and the stagnant film model. From experimental data using a singlestage system under total recycle conditions, mass balance models were used successfully to predict the performance of a multi-stage membrane system. A three-stage system gave 87% pure ALA and 83% pure BLG. Thus, dairy protein fractions of chromatographic purity can be obtained using charged ultrafiltration membranes arranged in stages, without the wastewater generation and expense of chromatography. The results of this study will be useful for expanding the utility of ultrafiltration membranes beyond simple protein concentration to fractionation of proteins. (C) 2013 Elsevier B.V. All rights reserved.