This study aims to evaluate the performances of the membrane processes during the treatment of blue whiting peptide hydrolysates. Ultrafiltration with high molecular weight cut-off (MWCO) can be used for the separation between peptides and non-hydrolyzed proteins. A membrane of MWCO 20 kDa was tested on hydrolysates containing 11.4 g of peptides per liter of solution. Steady fluxes were satisfactory (100 1/h/m(2) at 12 bars and 15 degrees C) but the nitrogenized matter retention is approximately 30%, which leads to a considerable peptide loss in the permeate. In addition, ultrafiltration with intermediate MWCO is considered for the enrichment of fractions of peptides of a definite molecular weight range. Two membranes of MWCO 4 and 8 kDa were selected; allowing easy separation between peptides of high and low molecular weight. Moreover, they show variable rejection rates according to classes of molecular weight, which gives hope for a possible fractionation in the range of 1000-7000 Da. Lastly, the nanofiltration enables the concentration of peptides of low molecular weight by avoiding an excessive concentration of salts. The results with a membrane of MWCO 300 Da are very encouraging because the retention of peptides bigger than 1000 Da is total so that the concentration of the retentate up to about 300 g/l can be reached. (c) 2007 Elsevier Ltd. All rights reserved.