This paper deals with the filtration of a single protein (P-lactoglobulin) solution in comparison with a mixture of yeast extract and P-lactoglobulin. It intends to show how all the compounds of a complex biological solution interfere with the results of filtration. Two different membranes have been used: one constituted from organic regenerated cellulose and the other from zirconium oxide. Both membranes are studied with regard to selectivity (Transmission rate) and fluxes (Resistance model). It is shown that the filtration of a single protein like P-lactoglobulin leads to the building of a dynamic layer at wall, the resistance and selectivity of which are conditioned by the interactions with the membrane support (specifically protein adsorption on inorganic material) and the characteristic behaviour of P-lactoglobulin (loss of solubility at its isoelectric point). But this layer is altered when yeast extract is added. These modifications result in flux decrease and changes in selectivity. Adsorption of peptides and ionic strength variations due to charged species contained in yeast extract are supposed to be involved in these modifications. It is thus demonstrated that the choice of a membrane for the filtration of a real biological fluid can not be done easily by only considering the filtration of one single compound. (C) 2002 Elsevier Science B.V. All rights reserved.