Protein fouling during microfiltration has been investigated for mixtures of bovine serum albumin (BSA) and lysozyme and of BSA and pepsin. Flux decay curves were analyzed using a recently developed model that accounts for simultaneous pore blockage and cake formation. The model is in good agreement with the data and can be used to evaluate the effect of mixture composition on the concentration of protein aggregates and the properties of the protein deposit. For pepsin-BSA mixtures, the initial fouling appears to be dominated by the BSA, whereas the rate of cake growth occurs primarily by the pepsin. This behavior is consistent with the large concentration of pepsin aggregates and the electrostatic repulsive interactions between the negatively-charged BSA and pepsin. The behavior is more complex for mixtures of BSA and lysozyme. In this case, the fouling is dominated by the lysozyme, although mixtures with small amounts of added BSA foul more slowly than observed with either of the pure proteins. (C) 2003 Published by Elsevier Science B.V.