The hydrodynamic permeability of porous membranes filled with cross-linked polyacrylamide (PAA) was measured as a function of the dimension less molecular size (ratio of molecular size to pore size). The gel was formed by first soaking the membrane (cellulose nitrate) in a solution containing acrylamide monomers (AA), cross-linking agent and initiator, and polymerizing in situ using the activation by ultraviolet irradiation. The permeability of the gel-filled membranes for protein molecules was examined with four kinds of protein solutions. The results were as follows. (1) The gel-filled membranes hinder protein permeation as a function of the ratio of protein size to pore size. (2) The AA concentration in the gel preparation greatly influences the pore size in the gel, and the pore size call be explained by a particle packing model. (3) The minimum pore size of the gel was 6.6 nm at the AA concentration of 50 kg/m(3). (4) The enriching processes of protein/enzyme solution were in good accordance with a theoretical equation derived from the assumption that the permeability of proteins through the gel-filled membranes can be simply expressed as a function of dimensionless molecular size.