Crossflow microfiltration is an efficient method for the clarification, stabilization and sterilization of fruit juices and other biological suspensions. One of the main problems with the application of this technique, however, is membrane fouling caused by the presence of proteins and polysaccharides. The fouling behavior of four 0.2 mu m hydrophilic microfiltration membranes (polysulfone, polycarbonate, polyvinylidene fluoride, and cellulose acetate) is described for protein mixtures of bovine serum albumin (BSA), lysozyme (LY), and ovalbumin (OV). The study of membrane fouling was carried out using a stirred cell, and then analyzed with electron microscopy and with flux decline, total resistance and permeate concentration versus time plots. During the filtration of single protein solutions using polysulfone and polycarbonate membranes, BSA and LY displayed only internal membrane fouling for a 3-h period, whereas OV exhibited an initial phase in which internal fouling dominated, followed by external or surface membrane fouling. When different binary protein mixtures were filtered through the polysulfone and the polycarbonate membranes, the highest membrane fouling was found for those mixtures containing ovalbumin, Finally, the filtration of a ternary protein mixture showed different fouling behaviors, depending on the surface porosities of the four membranes employed; lower surface porosities exhibited more rapid external membrane fouling.