Supramolecular redox-active assemblies on electrodes are of fundamental interest and can be used to create functioning devices such as sensors, biosensors, and bioreactors. The ability of redox-active films to mediate electron transfer reactions in 3-D dramatically increases the sensitivity with which target molecules can be determined. Metallopolyion hydrogel films immobilized on electrode surfaces exhibit many properties that are reminiscent of those shown by redox-active proteins. This review discusses the electrochemical properties and applications of such films, including mediating electron transfer between electrodes and oxidase enzymes. In addition, polyion-protein films grown layer by layer have certain advantages in device fabrication, including facilitating direct electron transfer for many proteins, mechanical stability, use of tiny amounts of protein, and control of film architecture. This review presents examples of iron heme proteins in films grown layer by layer by alternate electrostatic adsorption for catalytic reduction of hydrogen peroxide and trichloroacetic acid and for oxidation of styrene. (C) 2003 Elsevier Science (USA). All rights reserved.