The fabrication and operation of a microelectronic conductimetric biosensor is described. The device monitors the change in solution conductance occasioned by the catalytic action of enzymes immobilised over a planar conductance cell comprising serpentined and interdigitated metal conductor tracks. The output of the instrument was linear over a 3 min period on addition of urea to a sample cell overlaid with immobilised urease. The responses to any given urea concentration were reproducible to within approximately +/- 1%. The device responds to urea present in serum samples.