Abstract: The γ-toxin binding nicotinic acetylcholine receptor was isolated from the electric organ of Torpedo California (prep. described). A suspension of this material in phosphate buffer, pH 7 was mixed with γ-bungarotoxin, labelled with fluorescein isothiocyanate (I), in the presence of 0.1% Triton X-100 and set aside for 2 h at 4°C. The mixture was then centrifuged and the unbound I was determined by flow injection of 20 µL of the solution and measurement of the fluorescence at 525 nm (excitation at 490 nm). Other compounds which reacted with the same sites on the receptor inhibited the binding of I; the decreased binding was used to determine, e.g., pancuronium (II), d-tubocurarine, gallamine, carbamylcholine, decamethonium and hexamethoium (III). Depending on the binding constants, the working curves for these compounds ranged from 10^-5 to 10^-8 M (for II) to 10^-3 to 10^-5 M (for III).