A new immunoassay technique based on measurement of conductance changes in solutions is described. The assay employs an immobilized monoclonal antibody to capture a protein analyte along with a second antibody to the same analyte, conjugated to an enzyme capable of producing ions which are measured conductimetrically. Urease was selected as the enzyme, because it produces, from urea, four ions for each catalytic event. The analyte studied was human chorionic gonadotropin in serum. Higher concentrations of analyte during incubation with immobilized antibody and antibody-urease conjugate led to increased binding of the latter. After removal of unbound conjugate, urea solution was added and the rate of conductance change measured in the bulk substrate solution. Experiments, performed in polystyrene microtiter wells using a specially designed electrode, demonstrated the ability to measure 30 picomolar concentrations of human chorionic gonadotropin with a 30-s rate measurement. Urease proved to be an excellent labeling enzyme, retaining its activity under the nonionic conditions necessary to maintain low background conductance. Good agreement was obtained between observed rates and those expected from conductimetric theory and known physical parameters. The potential utility of the conductimetric immunoassay lies in the fabrication of biosensor devices for simplification and cost reduction of immunochemical-based instrumentation. Further improvements to the technique are proposed to achieve lower detection limits.