Flux decline and retention of aqueous solutions of several proteins (pepsin, BSA, lipase, gamma-globulin and invertase) with molecular weights of 36, 67, 80, 150 and 270 kDa are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 mu m made by Anopore" under a constant applied pressure of 5.0 kPa. The fouling process of this ultrafiltration membrane is investigated as a function of the protein concentration, the tangential velocity at the membrane surface and the nature and size of the protein. Finally, the deposition mechanism on the membrane surface and/or into its porous structure is analyzed in terms of several kinetic models. It is seen that an initial particularly intense flux decline due to external blockage is followed by an internal deposition (partially retained proteins) or the formation of a cake (totally retained proteins). The kinetics of these fouling mechanisms are proved to follow reasonable trends versus feed concentrations, recirculation velocity and the protein molecular weight.