The poor selectivity of membranes has been regarded as one of the critical factors limiting the application of membrane systems to protein fractionation. This study demonstrates that ultrafiltration enhanced by gas sparging, together with proper adjustment of solution conditions, can dramatically improve the selectivity of a commercially available tubular PVDF membrane (MWCO 100 kD) for the fractionation of the HSA/IgG mixture, as well as significantly increase permeate flux. For the system studied, the optimal solution condition was found to be at pH 8 and salt concentration of 40 mM. In this system the transmission of IgG was much higher than that of HSA, i.e. a. reversed selectivity was observed. The results were explained on the basis of the balance of the hydrodynamic and electrostatic forces acting on the molecules when they approach and enter the membrane pores. The effect of other operating parameters, including TMP, air and liquid flowrates and protein feed concentration, were also investigated and the optimal conditions were identified. With gas sparged ultrafiltration under those conditions, almost complete separation of the two proteins was achieved. Finally, the possible damage to proteins induced by air bubbles was studied and it was concluded that the damage was negligible under the selected operation conditions.