Normal-flow filtration is used frequently in the biopharmaceutical industry for applications such as sterile filtration of fermentation media, buffers and product proteins. Though the membrane pores are much larger than the product protein, protein fouling of the membrane often leads to a decrease in permeate flux at constant feed pressure. Normal-flow microfiltration experiments were conducted using polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) base membranes, PTFE membranes coated with polyvinyl alcohol (PVA) and Teflon AF, and PVDF and polycarbonate membranes coated with PVA and polyvinyl pyrrolidone. Feed streams consisted of bovine serum albumin (BSA) and hemoglobin (Hb). Feed pH values of 4.7 and 7.2 and pressures of 14 and 69 kPa were used. BSA concentrations of 4.0 and 0.1 g L-1 were tested. For Hb, concentrations of 2.0 and 0.1 g L-1 were tested. Protein fouling of the membrane was investigated by determining the variation of permeate flux with filtrate volume and by analysis of ATR-FTIR spectra and FESEM images of virgin membranes and membranes after microfiltration. Quantization of bound protein mass was done by carrying out equilibrium adsorption measurements. Results indicate that fouling by Hb is greater than BSA. Fouling is more severe at higher protein concentration and feed pressure. Increasing membrane hydrophilicity by coating with PVA decreases the amount of fouling. Our results indicate that it is the interplay among membrane properties, protein properties and operating conditions that determine whether fouling occurs and to what extent. (C) 2009 Elsevier B.V. All rights reserved.