A monoclonal antibody raised against the muscarinic acetylcholine affinity-alkylating antagonist propylbenzilylcholine mustard was tested for its ability to recognize affinity-alkylated muscarinic receptors. We demonstrate here that although the antibody will not recognize the mustard when it is covalently linked to the native muscarinic receptor, trypsinization of affinity-labeled membranes releases a proteolytic labeled fragment that can be specifically immunoprecipitated by the antibody. Electrophoretic analysis of the immunoprecipitate indicates that the ligand was associated with a polypeptide of molecular weight 5,000. The recognition of this fragment by the antibody provides a means to immunopurify a portion of the muscarinic receptor that is at or near the ligand binding site.