Ultrafiltration of proteins and other compressible material is characterized by a significant transmembrane pressure and is often limited by the formation of a fouling layer. This study examines the compressive rheology of such fouling layers. A model is proposed that can. be used to predict the performance of filtration processes where there is significant pressure drop across the clean membrane and the filtration rate is limited by fouling during the filtration cycle. The information required for the model can in principle be obtained from simple centrifuge tests and the clean membrane pressure drop. The model can be used to optimize the cycle times, pressures, and other operating parameters of the ultrafiltration process. The model was tested oil the ultrafiltration of whey protein concentrate and shown to describe the experimental data. (C) 2004 American Institute of Chemical Engineers.