Ultrafiltration of bovine serum albumin solutions using cellulose triacetate (CTA) and regenerated cellulose (RC) membranes showed that CTA membranes were more vulnerable to fouling than hydrophilic RC membranes. The flux decline for both membranes, which was more significant initially, could be attributed to concentration polarization near membrane surfaces and adsorption of the protein. Fouling of the membranes was characterized by the resistance of protein deposits to the permeation of pure water through fouled membranes. The CTA membranes gave more resistant protein deposits than the RC membranes and the resistance increased, two to three times, in the presence of salts but decreased substantially at higher temperatures.