Flux and retention of 0.1% w/w aqueous solutio ns of several proteins [lysozyme, pepsin, bovine serum albumin (BSA), lipase, and gamma-globulin] with molecular weights of 14.6, 36, 67, 80, and 150 kDa are studied when they are tangentially filtered, with transmembrane pressure differences until 1 MPa and circulation velocities in the retentate loop from 0.04 to 1.98 m/s (laminar regime), through two asymmetric polysulfone commercial membranes (E-100 with a nominal pore size of 0.01 mu m and E-500 with a nominal pore size of 0.04 mu m). Results are analyzed with the film theory for the concentration-polarization phenomenon, obtaining the mass transfer coefficient along with the apparent and true retention coefficients for the cell used, as a function of the feed circulation velocity and the molecular weight of the solute. The standard retention curves lead to pore size distributions differing from the nominal ones. These differences can be attributed to the modifications of the membranes when they are in operational conditions, probably due to protein adsorption.