It was studied how protein-protein and protein-membrane interactions influence the filtration performance during the ultrafiltration of protein solutions over polymeric membranes. This was done by measuring flux, streaming potential, and protein transmission during filtration of bovine serum albumin (BSA) solutions at various pH values using various membranes with different cut-off values. It was found that protein-membrane interactions influence the fouling behaviour in the initial stages of filtration. In the high-fouling regime (later stages of filtration), protein-protein interactions dictate the overall performance. AFM images of the membrane surfaces taken after the filtration experiments showed that the membranes were totally covered by a protein fouling layer. The structure of this fouling layer depended strongly on pH. In particular, very open structures with high permeabilities were found at low pH (below the iso-electric point of the protein). These induced high values of flux and protein transmission.