The disclosed in-vitro assay method can test for a wide variety of drugs, toxic substances, etc. which have affinity for a chemoreceptor of a living organism. The chemoreceptor is provided in vitro as the recognition site for the drug, toxin, etc. in the analyte or sample to be assayed. Before bringing the analyte into the presence of the chemoreceptor, a known amount of enzyme-labelled ligand for the chemoreceptor is provided, and the binding of the ligand to the chemoreceptor inactivates or inhibits the enzyme "label" of the enzyme-labelled ligand. Accordingly, when the analyte contacts the chemoreceptor, a competitive situation is created; the ligand and the analyte are now in competition for the binding sites of the chemoreceptor, and at least some of the previously-bound, enzyme-labelled ligand becomes excluded from the binding sites, resulting in an increase of observable enzyme activity in the system. Determination of this increase in activity provides a detection or quantitative measure of the presence of drug, toxin, etc. in the analyte. If desired, several drugs, toxins, etc. can be tested for at the same time in this method; moreover, even novel or unknown toxins or drugs can be detected and their concentration quantified. Stated another way, this assay combines the "enzyme-multiplification" effect of the enzyme-multiplied immuno technique ("EMIT") with the biological sophistication of a receptor-based assay.
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