Abstract We propose a novel class of biosensors based on membrane bound receptors or transport proteins as the sensing element. The protein is incorporated in a planar lipid bilayer which covers the transducer. The transducer may detect an electric current, a voltage, or a change in fluorescence. A prototype lactose sensor is presented which consists of a quartz slide covered by a lipid membrane containing the protein lactose permease from Escherichia coli. This protein is a lactose/H + contrasporter, hence lactose in the external medium initiates lactose/H + cotransport across the lipid membrane. This leads to a rise in proton concentration in the small volume between the lipid membrane and the quartz surface which can be detected by a pH-sensitive fluorescence dye.