Novel multifunctional membranes were prepared by ultraviolet photopolymerization of N-isopropylacrylamide (NIPAAm) on the surface of porous hydrophilic polypropylene microfiltration membranes. The polyN-isopropylacrylamide PNIPAAm gels were generated on the surface of the membrane through a covalent bond in the presence of a crosslinking agent, N,N＇-methylenebisacrylamide. The crosslinked PNIPAAm gels are temperature-responsive hydrogels that can swell and deswell reversibly in aqueous solution around the vicinity of the lower critical solution temperature (LCST) of PNIPAAm. With a change of temperature, the effective pore size of the membrane surface can be enlarged or shrunk as the PNIPAAm gels swell or deswell. Above the LCST of PNIPAAm, the fluxes of water and solution containing 500 ppm of dextran (molecular weight: 1.67 x 10(5) g/mol) through the temperature-sensitive membranes are about 6 and 85 times higher than those below the LCST of PNIPAAm, respectively. The changeable flux makes it possible to employ the temperature-sensitive membranes as a sensor or valve that regulates filtration properties by responding to temperature. Solutions of dextran with a molecular weight from 6300 to 2,000,000 were used to evaluate the separation performance of the temperature-sensitive membranes as the ultrafiltration membrane. The ranges of rejection of dextran and the flux of solution are from 0 to 90 and 8 to 32 l/m(2) h, respectively, depending on the temperature, pressure, and molecular weight of dextran. It is clear that the temperature-sensitive membrane exhibits multifunctional characteristics; that is, the microfiltration membrane is above the LCST of PNIPAAm, and the ultrafiltration membrane is below the LCST of PNIPAAm.