Protein fractionation of lysozyme and myoglobin mixtures was investigated using sandwich arrangements of ultrafiltration membranes under controlled permeate flux. Both proteins have close molecular weights with values of 14300 g mol(-1) for lysozyme and 17400 g mol(-1) for myoglobin. The aim was to improve the selectivity of the separation by masking the pore size distribution of ultrafiltration membranes by having one membrane on top of another. Two membranes were arranged together in sandwich configurations. Two possible sandwich configurations were compared by assessing the transmission and selectivity for the protein mixture. For both sandwich configurations membrane fouling was reduced compared with a single membrane. Overall the S-sandwich with Biomax PES membranes was superior. (C) 2007 Elsevier B.V. All rights reserved.