Fractionation of a beta-lactoglobulin (beta-LG) peptide mixture by nanofiltration (NF) membranes was investigated. Peptide mixture was prepared by tryptic hydrolysis of commercial beta-LG followed by ultrafiltration (UF) for enzyme removal. The peptide mixture (permeate) was further fractionated using an Osmonics-SG13 cellulose acetate NF membrane with a molecular weight cut-off (MWCO) of 2500g mol(-1). The effect of hydrodynamic parameters on a "resistance-in-series" model and transmission of tracer peptides was studied at 0.5 wt.% hydrolysate concentration by varying the operating pressure (0.200 and 0.500 MPa), recirculation rate (0.17 and 0.29 m s(-1)), and recirculation time (30 and 120 min). A detailed comparison, on individual peptide transmission revealed that increasing recirculation time to 120 min, at lower recirculation rates, greatly affected peptides beta-LG 102-105 and beta-LG 142-148 at both operating pressures. Our results are indicative of important changes in NF selectivity as affected by hydrodynamic conditions and recirculation time via possible peptide-peptide interactions occurring in the so-called weakly attached layer.