Factors affecting protein fouling of ultrafiltration membranes were investigated both in the presence and absence of a net transmembrane flux. The results obtained for the zero flux system showed that membrane-associated protein levels followed a saturation type isotherm having a maximum value of 207μg-cm −2. In the presence of flux, membrane-associated protein showed an increase with increasing transmembrane flow. Under the conditions studied the membrane-associated protein eventually reached a flux-independent concentration which was a function of the tangential recycle flow. In all cases the limiting protein level was higher than the maximum level obtained in the absence of flux. The flow resistance of the fouling layer continued to increase after the membrane-associated protein had reached a limiting value, supporting the concept of a limited flux approach to modelling. The protein “adsorption” observed was considerable higher than could be explained by monolayer formation, and unlike Langmuir adsorption was shown to be freely reversible with changes in the hydrodynamic conditions.