We have measured fluorescence resonance energy transfer (FRET) between a fluorescent antagonist, bound to the purified detergent-solubilized serotonin type 3 receptor, and a lipophilic acceptor probe partitioned into the micelle surrounding the detergent-solubilized receptor. The exptl. obsd. FRET efficiency was evaluated on the basis of the characteristic dimensions of the receptor-micelle complex and the av. no. of acceptor mols. in such micelles. The binding site was detd. to be 5.4 +- 0.9 nm above the center of the detergent micelle. The expts. were performed below the crit. micellar concn. of the detergent (C12E9) used to solubilize the receptor, under which conditions it was demonstrated that the ligand binding activity was fully preserved. This reduces considerably the fluorescence background arising from probes not assocd. with the receptor, allowing a precise detn. of the transfer efficiency. [on SciFinder (R)]