Isolated acetylcholine receptors from Torpedo nobiliana are used as recognition elements for phencyclidine (PCP), a potent hallucinogen with high affinity for the ion channel site associated with the receptor, in a novel enzyme-amplified receptor assay. A fixed amount of PCP labelled enzyme competes with free PCP for a limited number of receptor sites resulting in changes of observed enzyme activity proportional to free drug concentration. The proposed concept combines the use of receptors with an enzyme amplification step to yield an exceptionally simple and convenient assay.