The aim of this study was to present a new concept of site-directed reduction of disulfide bonds based upon the use of an affinity ligand harbouring a readily oxidizable dithiol. The cysteine bond involved in the acetylcholine binding site of the AChoR was specifically reduced by a carbamylcholine analogue. The ligand, in its oxidized form, was characterized by an affinity constant of 20 microM for the agonist binding site. In its dithiol form, it specifically reduced the disulfide between Cys-192 and Cys-193 on the alpha-subunits of the nicotinic acetylcholine receptor. This reduction needed 10 times lower concentration when carried out with site-directed reducing agent (ARA) than with DTT, and was highly specific for the alpha-subunits. The contribution of the carbamylcholine moiety of the site-directed reducing agent was clearly demonstrated in kinetic studies where reduction abilities of ARA, DTT and the methylated analogue of ARA (MeRA) were compared. At the same concentration (20 microM), DTT and MeRA had a 25 times lower initial rate of reduction than ARA. With 200 microM of DTT this initial reduction was still 4 times lower. Furthermore, the use of a maleimido undecagold cluster which specifically labeled the reduced nicotinic receptor opens the way to structural analysis of the agonist binding site by electron microscopy. These results demonstrate the potency of this kind of site-directed reducing agent for structural study of receptors or enzymes involving a disulfide bond in their active site.