The aim of this study was to quantify the specific effect of adsorption on membrane fouling during filtration of a membrane bioreactor (MBR) mixed liquor suspension. Adsorption experiments were performed on well-defined protein solutions (beta-lactoglobulin solutions) to provide reference results and compare them to those obtained during the filtration of MBR suspensions (raw suspension and settled suspension). Two different methods were used to quantify the role of adsorption in membrane fouling: a "static" method in which membranes were immersed in the biological suspension and a "dynamic" method supposing that the resistance due to adsorption is an irreversible phenomenon that remains after filtration and back-washing. It was shown for the two types of suspensions that (i) due to limited diffusion, the dynamic method appears to be more adapted than the static methods (ii) adsorption is a rapid fouling phenomenon that induces irreversible resistance and that. in frontal mode takes place at the beginning of the operation: (iii) the adsorption phenomenon shows specific hydraulic resistance of the same order of magnitude as the clean membrane resistance: (iv) other phenomena, i.e. progressive pore clogging, can also take place though subcritical hydrodynamic conditions.