The flux decline, is studied in typical experiments with dead-end microfiltration of BSA solutions (1, 3, 5 and 10 g/L) through Cyclopore track-etched polycarbonate membranes (nominal pore size 0.1 mu m) at several pH values and two ionic strengths. Results have been analyzed in terms of the common blocking laws and correlated with the operation parameters. Variations of pressure, concentration, pH, and ionic strength have shown great influence on the kinetics of protein deposition. In any case, the process of membrane fouling can be divided in two steps, clearly separated in all the experiments : a rapid initial internal blocking, strongly dependent on operation parameters, and a final stage of external blocking with lower sensitivity of the flux behavior on operation conditions. Finally, the amount of adsorbed protein and its influence on pore size distribution have been analyzed by desorption with a SDS-solution and by an extended bubble point method. These results show that the initial internal pore blocking can be attributed to protein adsorption while the long-time fouling should be caused mainly by solute-solute interactions.