Transmission of beta-CN (193-209), a hydrophobic peptide derived from C-terminal part of bovine beta-casein, during crossflow ultrafiltration through M5 Carbosep membrane (carbon-zirconium) was studied as a function of pH, ionic strength and in the presence of small peptides mixture used as adsorption competitors. It was shown that part of the high apparent rejection observed seems to be due to the hydrophobic interactions between peptide and membrane support. Hence, using three membrane types : M5 Carbosep (carbon-ZrO2, 10 kDa (MWCO)),M2 Carbosep (carbon-ZrO2, 15 kDa) and Kerasep (Al2O3-ZrO2, 15 kDa), the influence of the supports was investigated in relation to the effects they produce on beta-CN (193-209) transmission and adsorption. At steady state of the process, the transmissions obtained were 6, 53 and 66% while the percent of the peptide adsorbed were 41, 89 and 0% with M5, M2 and Kerasep membranes, respectively. These data indicate that, in the case of Carbosep membranes, the combination of peptide retention within the membrane pores and/or on the upper membrane surface as well as the adsorption of the solute on the carbon support were at the origin of the low unexpected beta-CN (193-209) concentration in the permeate side.