The stability of several polymeric ultrafiltration membranes in aqueous ethanol was evaluated with an ethanol-soluble protein. Concentration polarization effects were observed at concentrations of 5-150 g/L of the protein, with flux becoming independent of pressure above 100-200 kPa. The data followed the film theory, resulting in a C. value of 340 g/L with the model protein. Protein rejections for the-selected membranes were 80-95%. However, even with prior conditioning, some membranes (polysulfone, polyacrylonitrile, and cellulosic) that initially appeared to give good performance and stability failed over a period of 10 weeks, resulting in an increase in flux or decrease in protein rejection. Polysulfone hollow fibers gave good initial performance but degraded in less than 24 hours of cumulative use.