Biosensors combine a biological recognition mechanisms with a physical transduction technique. In nature, the transduction mechanism for high sensitivity molecular detection is modulation of cell membrane ionic conductivity, through specific ligand - receptor binding induced switching of ion channels. This effects an inherent signal amplification of 6-8 orders of magnitude, corresponding to the total ion flow arising from the single channel gating event. Here we describe the first reduction of this principle to a practical sensing device, which is a planar impedance element composed of a macroscopically supported synthetic bilayer membrane incorporating ion channels. The membrane and ionic reservoir are covalently attached to an evaporated gold surface. The channels have specific receptor groups attached which permit switching of the channels by analyte binding to the receptors. The device may then be made specific for the detection of a very wide range of analytes, including proteins, drugs, hormones, antibodies, DNA, etc., currently in the 10-7-10-12 M range. It also lends itself readily to microelectronic fabrication, the optimum sensitivity range of the device may be tuned over several orders of magnitude.