To compare actual and effective porous structure in operative conditions of ultrafiltration membranes, flux, retention, and the amount of adsorbed protein have been measured for 0.1% w/w aqueous solutions of several proteins [lysozyme, pepsin, bovine serum albumin (BSA), lipase, and gamma-globulin] with molecular weights from 14.6 to 150 kD tangentially filtered through two asymmetric polysulfone membranes, E-100 and E-500. From retention and flux experiments, the dependency of mass transfer coefficients on molecular volume has been analyzed. Results imply that protein molecules behave as being slightly uncoiled, especially when filtered through the smallest pore size membrane. By using a simple sieving model, retention data allow pore size distributions to be obtained. The data are modified by taking into account adsorption and volume hindrance effects in operational conditions.