Communication: An amperometric biosensor for glucose, based on an immunological recognition process between an antigen binding fragment (Fab, see Figure), immobilized in a planar bilayer lipid membrane, and glucose oxidase. The enzyme-catalyzed formation of H2O2 in the presence of glucose can be detected amperometrically by means of an electrode. The principle of this sensor is also applicable for sensing a wide range of other substrates.