Fish protein hydrolysis can produce peptides of biological interest such as those possessing antioxidant, antiradical, anti-cancer, anti hypertensive, opiate-like. antiviral or secretagogue activities. Hydrolysis of "white fish" proteins is carried out according to two distinct procedures to obtain interesting biological activities. The aim of the study is to split peptide hydrolysates by using a sequence of membrane filtrations in order to isolate fractions enriched in peptides of specific molar mass. The most bioactive peptides classes identified are mainly in the range [100-500] and [1000-3500] Da. The distribution profile in molar mass of the peptides contained in the various fractions is obtained by size exclusion chromatography. Then, a process associating ultrafiltration and nanofiltration is proposed. The performance (flow, retention) of each technique is studied individually and the efficiency of the combined process for enrichment in molecules of specific molar mass is evaluated. (C) 2009 Elsevier Ltd. All rights reserved.