A new receptor based assay is described for the determination of classes of drugs which have high affinities for the acetylcholine receptor. The method is based upon the inhibition of the enzyme activity of an enzyme-drug conjugate by the binding to the receptor protein, and competition between free drugs and the enzyme-drug conjugate for a limited number of receptor sites. The activity of the enzyme marker system, glucose-6-phosphate dehydrogenase covalently conjugated to desipramine, is monitored by colorimetric detection of the rate of NADH formation at 340 nM. The procedure proposed is designed to provide a simple drug screen which can be done in a minimally equipped laboratory while achieving the required sensitivity. The technique is illustrated for three acetylcholine channel binding compounds: the hallucinogen phencyclidine (PCP) and the antipsychotic agents chlorpromazine and trifluoperazine. This procedure yields calibration curves with detection limits at nanomolar levels of drug, with binding responses dependent on the amounts of receptor and enzyme-labeled drug used. Aspecific binding responses of unlabeled enzyme to drug or receptor to compounds with low affinity for the receptor are shown to have minimal effect on the assay.