Ultrafiltration experiments were performed on mixtures of BSA and lysozyme with the aim of fractionating the two proteins. Pilot-plant scale polysulphone hollow fiber membrane modules with molecular weight cutoffs of 150 000 and 200 000 were used. The membrane process was enhanced via the injection of gas slugs into the hollow fiber membrane modules. The gas slugs served to disrupt concentration polarization near the membrane surface with the subsequent aim of improving membrane selectivity. It was found that hollow fiber membranes were effective at fractionating BSA and lysozyme, with the membrane selectivity being greater than 20 across a range of operating conditions. The introduction of gas slugs further improved membrane selectivity by a factor of 3-5. The sieving mechanism of the membrane and electrostatic interactions between proteins and the membrane were discussed and used to explain experimental observations.