Abstract: The invention relates to a novel assay for the determination of entities having biological activity and to a device for carrying out such determinations. The assay is a very sensitive one and quantities in the picogram per milliliter range can be determined. The assay is based on chrono-coulometric measurements with sequential measurements of a plurality of samples being carried out with the aid of a multiplexer, in combination with a potentiostat, there being provided suitable electrodes and means for applying a predetermined voltage. One of the electrodes is advantageously a glassy carbon electrode, carbon felt or cloth or carbon paper. Amongst biologically active entities there are antibodies/antigens; hormones/receptors; nuclotides/nucleotide probes, one of the members of such a pair being immobilized on an electrode surface, being tagged with an enzyme providing a signal in coulometric measurements.