The detergent sodium cholate was used to both solubilize and partially delipidate the nicotinic acetylcholine receptor from Torpedo californica. Using both native membranes and reconstituted membranes, it is shown that the detergent to lipid molar ratio is the most important parameter in determining the effect of the detergent on the functional properties of the receptor. Receptor-lipid complexes were quantitatively separated from detergent and excess lipids by centrifugation through detergent-free sucrose gradients. The lipid to protein molar ratio of the complexes could be precisely controlled by adjusting the cholate and lipid concentrations of the starting membranes. Analyses of both ion influx activity and ligand binding revealed that a minimum of 45 lipids per receptor was required for stabilization of the receptor in a fully functional state. Progressive irreversible inactivation occurred as the lipid to protein mole ratio was decreased below 45, and complete inactivation occurred below a ratio of 20. The results are consistent with a functional requirement for a single shell of lipids around the perimeter of the receptor.