It has been observed that bovine serum albumin (BSA, 69 000 Da) exhibits high rejection in a pH 7.4, 0.15 M NaCl solution containing low concentrations of immuno-gamma-globulins (IgG, 155 000 Da). Although it is apparent that this phenomenon is likely due to the complex interactions of the proteins as well as to simple steric hindrance, it is not clear to what extent these resistances contribute to the solute flux loss. This study investigates how variations in IgG concentration, solution pH, and ionic strength can affect the solute and the permeate fluxes for the ultrafiltration of BSA through 100 000 MWCO cellulosic membranes in a batch cell. The results showed that, unlike cases performed at pH 7.4 in 0.15 M NaCl, the presence of IgG may increase the transport of BSA under certain conditions. This study does show the potential value and insights that will result from further binary protein ultrafiltration research.