Mucin glycoproteins (mucins) are the major macromolecular constituent of the mucosal gel which lines mammalian epithelial tracts and bestow physical and biological properties to mucus. The multifunctional nature of these large, highly O-glycosylated macromolecules reflects structural determinants in both their protein backbones (encoded by MUC genes) and O-glycosidic oligosaccharides. Elucidation of the role of mature, i.e. O-glycosylated, mucins in airway health and diseases requires identification and delineation of both the MUC protein backbone and the O-glycosidic oligosaccharide chains covalently attached to each MUC backbone. The relative levels of mature MUCs in airway secretions is not known, although seven of the nine currently identified human MUC genes are expressed in human airways. Tracheobronchial mucin (TBM), a glycosylated gene product of MUC5, has been isolated as a major component from lung mucus of a patient with bronchial asthma. In order to elucidate the role of TBM in airway health and disease, we have raised peptide-specific polyclonal antibodies to the TBM:TR-3A domain of MUC5 and isolated monospecific antibodies by affinity chromatography. The monospecific antibodies immunoprecipitated MUC5 both as the protein precursor and MUC5/TBM as the glycosylated mucin from secretions of A549 cells, a lung adenocarcinoma cell line. Utilization of peptide-specific antibodies will provide a basis for evaluating disease-induced alterations by allowing isolation of mucins with specific MUC backbones from healthy and pathological secretions. This work was funded in part by NIH RO1 HL33152 to MCR and a CFF student traineeship to HJG.