The highly selective 5-HT3 receptor antagonist [H-3]GR65630 has been used to characterize 5-HT3 receptors in intact N1E-115 neuroblastoma cells. Equilibrium binding analysis demonstrated high-affinity binding to a single class of receptors with a K(d) of 0.69 (+/- 0.12) nM and B(max) of 31.4 (+/- 11.4) fmol/10(5) cells, equivalent to approximately 200 000 sites per cell. Specific binding was displaced by low concentrations of 5-HT3-selective ligands, and by the nicotinic antagonist d-tubocurarine.