The flow and retention of 0.1% w/w aqueous solutions of pepsin, bovine serum albumin (BSA), lipase, gamma-globulin, and invertase with molecular weights of 36,000, 67,000, 80,000, 150,000, and 270,000 dalton (g/mol) are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 x 10(-6) m made by Anopore, with transmembrane pressure differences up to 100 kPa. The data were analyzed within the frame of the film layer theory for the concentration polarization phenomenon. This allows the mass-transfer coefficient to be obtained for the cell used as a function of the feed circulation speed and the molecular weight of the solute. Apparent and true retention curves obtained lead to a size exclusion radius smaller than the nominal one but very similar to that obtained from scanning electronic photographs.