a-Bungarotoxin is one of a class of proteins, isolated from snake venoms, which antagonize the action of acetylcholine at vertebrate neuromuscular junctions and ‘electroplaques' of electric fish. a-Bungarotoxin blocks acetylcholine action irreversibly and may be labelled with either 125 I or 3 H. This irreversible binding is used as the basis of an in vitro assay for acetylcholine receptors, whether in intact tissue, membrane fragments or solubilized preparations. Acetylcholine receptors from Torpedo and denervated skeletal muscle have been solubilized and substantially purified using affinity chromatography. The distribution of acetylcholine receptors in several tissues has been determined, and an auto-immune response, induced by injection of purified Torpedo receptors, has been studied.