[3H]Choline mustard aziridinium ion binds irreversibly to the sodium-coupled high-affinity choline transport protein in a sodium-dependent and hemicholinium-sensitive manner, and thus is a useful affinity ligand. In rat striatal synaptosomal membranes, it radiolabels two polypeptides with apparent molecular masses of 58 and 35 kDa. Based upon the use of two different experimental approaches, it appears that neither of these polypeptides is glycosylated.