Abstract: A protein-immobilizing hydrophobic polymeric membrane comprising an organic, hydrophobic polymeric substrate with a specific protein reactive group pendant therefrom in preselected concentration is disclosed. The protein reactive groups is a free group of a hydrocarbon chain which has one end thereof absorbed into the surface of said polymeric substrate; said free group, generally at the end of said chain, covalently bonding with a protein molecule. The concentration of protein reactive sites is sufficiently low that the polymeric substrate retains its hydrophobic character. An electrode coated with a protein-immobilized membrane, i.e., a membrane having a hydrocarbon chain to which a protein molecule is attached, provides, in conjunction with a reference electrode, an electrically sensitive system for measuring change in concentration of a compound in solution where said compound selectively reacts with the immobilized protein.