Selective transmission of a solute through membranes proves to be a challenge in ultrafiltration processes. This is because the transport of a solute through an ultrafiltration membrane does not depend on size alone, but on several other factors such as solute-solute and solute-membrane interactions. By manipulating physicochemical parameters and process variables (eg. pH, ionic strength, concentration of solute, etc.) and by membrane modification, it is possible to enhance the transmission of a particular solute and thus enhance fractionation of solutes. In this paper, the effect of pH on fractionation of BSA and lysozyme by ultrafiltration through 50 kDa MWCO (molecular weight cut off) polysulfone membrane has been examined. It was found that the selectivity of solute separation for dilute mixtures of BSA and lysozyme was very much pH dependent and varied from 3.3 at pH 5.2 to 220.0 at pH 8.8. However, at a higher feed concentration, the transmission of lysozyme through polysulfone membrane decreases quite dramatically resulting in lower throughput of product. An attempt has been made to enhance the transmission of lysozyme through the polysulfone ultrafiltration membrane by pretreating the surface of the membrane by adsorption of another protein, myoglobin. An increase in lysozyme transmission of up to 63% with respect to native membrane was observed. The stability of this pretreatment and its effect on permeate flux have been examined. The pretreated membrane was used to fractionate BSA/lysozyme mixtures. Even at higher feed concentration, enhanced fractionation with respect to native membrane was observed due to highly enhanced transmission of lysozyme through the pretreated membrane.