Human saliva contains high and low molecular weight mucin glycoproteins, that are distinct. Recently the gene encoding low molecular weight salivary mucin was cloned and designated MUC7, whereas the primary structure of high molecular weight salivary mucin is unclear. Furthermore, the expression patterns of high and low molecular weight salivary mucins in salivary glands have been debated. We have previously generated monoclonal antibodies specific for the peptide cores of salivary mucins. In the present study a monoclonal antibody specific for high molecular weight salivary mucin was used to screen a human salivary gland cDNA library. A single clone, SAL1, was identified and found to be encoded by tracheobronchial mucin gene MUC5B. A previously reported partial cDNA sequence from salivary mucin was linked to SAL1/MUC5B by genomic cloning and reverse transcriptase-polymerase chain reaction. Northern analysis of salivary gland RNA probed with SAL1 suggested that MUC5B was highly expressed in salivary glands. In situ hybridization was performed with a SAL1/MUC5B probe and a MUC7 probe. All mucous cells from the submandibular, sublingual, palatine, and labial glands labeled with the MUC5B probe, while serous cells labeled with the MUC7 probe. These findings were in accordance with our previous immunohistological results of the cellular localizations of salivary mucins. The results suggest that MUC5B is identical to or a major fraction of high molecular weight salivary mucin, and that MUC5B is expressed in all mucous cells of salivary glands. In contrast MUC7 is expressed in serous cells of salivary glands except the parotid glands.