A novel method of fabricating pore-filled membranes has been developed by coating a hydrophobic polyolefinic microfiltration membrane with polyethylenimine (PEI) to produce a chemically active surface. An evaporative coating technique was used to coat the internal surface of a polyolefinic membrane with PEI in chloroform (CHCl3). The coated PEI was then crosslinked by naphthalene-1,4-disulfonyl chloride in carbon tetrachloride (CCl4). The incorporation of the PEI coating changed the properties of the base membrane from hydrophobic to relatively hydrophilic; the nitrogens could be titrated, and the coating could not be washed out. 4,4 ' -Azo-bis(4-cyanovaleryl chloride), a heat-sensitive radical source, was incorporated into the coated membrane. The coated membrane, with incorporated azo compound, was heated to 75 degreesC in the presence of acrylic acid, 4-(vinylpyridine), or styrene to give the corresponding grafted, pore-filled membranes. The membranes exhibited hydraulic fluxes and pH valve effects consistent with molecular brushes grafted within the pores of the membranes.