Fractionation of proteins by ultrafiltration is an interesting challenge. With a particular membrane, the protein-protein and protein-membrane interactions largely decide which protein passes through and which is retained. System hydrodynamics also affects protein fractionation as the transmission behavior of a protein is altered by concentration polarization. Therefore, disruption of the concentration polarization layer would help to maintain the native selectivity of the membrane and hence aid in protein fractionation. An attempt is made to use gas sparging, a technique proven effective in controlling concentration polarization to enhance the selectivity of separation of BSA (MW 67,000) and lysozyme (MW 14,100). The effects of gas flow rate, liquid flow rate and feed concentration on the selectivity of fractionation are examined. Gas sparging enhances protein fractionation; under suitable solution conditions, nearly complete separation of BSA and lysozyme was achieved with gas sparged ultrafiltration. The permeate flux was also increased by gas sparing. The mechanism of enhancement is explained in terms of disruption of the concentration polarization layer and enhanced mass transfer due to bubble-induced secondary flow.