The fractionation of aqueous solutions of bovine serum albumin (BSA) and Dextran-T10 in a hybrid membrane cell is studied by numerical methods. The hybrid membrane cell has n semipermeable and n fully permeable membrane subsections alternating in series. The cell has a feed stream and three outlet streams: the retentate stream, a BSA-rich permeate stream, and a dextran-rich permeate stream. The flow and concentration fields of the solutes are determined by solving the Navier-Stokes and mass transport equations. The apparent selectivity of the separation process is determined from the concentrations of the solutes in the permeate streams. The selectivity of a hybrid membrane cell is compared with the selectivity of a conventional membrane cell. The selectivity is higher in the hybrid membrane cell due to the depolarization effect promoted by the fully permeable membranes. The difference is significant in the range of transmembrane pressure for which the membrane of the conventional cell becomes polarized. The selectivity of the hybrid membrane cell is maximized for high permeate velocities through the fully permeable membranes and high number of membrane sections. Additionally, the less transmitted component is preferentially recovered in the stream that crosses the fully permeable membranes. In comparison with conventional cells, the hybrid membrane cell has good performance along a large range of the transmembrane pressure, a characteristic that gives it good operational flexibility.