Membrane fouling by protein mixtures during microfiltration has been investigated for binary protein mixtures, including BSA and lysozyme, BSA and casein, and BSA and ovalbumin. Filtration experiments were carried out using 0.2 μm polycarbonate track-etched (PCTE) membrane in a stirred cell under constant transmembrane pressure (14 kPa) to study the effect of solution pH and mixture composition on filtrate flux decline. Flux decline data were analyzed using a recently developed pore blockage-cake formation model. The model was in a good agreement with the data. Protein mixtures may foul more slowly or faster than either of the pure proteins. The rate of flux decline for BSA-lysozyme mixtures is much greater than either of pure proteins. The flux increases as the percentage of lysozyme in the mixture increases. However, the initial flux of BSA-casein mixtures was observed to decrease with increasing casein concentration partially due to the rapid casein adsorption on the membrane surface. It can be concluded that casein plays an important role in the flux decline of protein mixtures. And, the flux decline of the BSA-ovalbumin mixtures increases as BSA increases.