The recovery and concentration of proteins from the waste water of a fish plant was achieved by ultrafiltration. Two UF modules equipped with Ceraver and Patterson Candy International (PCI) membranes were tested. Despite different cut-off values, similar apparent rejection coefficients (70% and 80% respectively) were obtained. Optimum economic conditions were established, corresponding to average transmembrane pressures of 2.2 x 10(5) and 3.8 x 10(5) Pa and tangential flow rates of 6.0 and 0.47 m s(-1) for Ceraver and PCI membranes, respectively. The protein concentration in the feed solution was increased from 5 to 35 g dm(-3). The study showed that the method could reduce pollution due to organic matter by decreasing the value of the Biological Oxygen Demand after 5 days (BOD5) by about 80%.