In recent years considerable advances have been made in our knowledge of the peptide moiety of human mucins through cDNA cloning. In many diseases disorders in mucin biosynthesis are observed, which result either from changes in the synthesis of the carbohydrate side chains or from differences in the relative expression of the different apomucins, each of which may affect physical properties of the viscous gel. We describe in situ hybridization studies on healthy human mucosae with five different oligonucleotide probes corresponding to each of the human genes known to date that encode secreted mucins, i.e., MUC 2, 3, 4 (HGM nomenclature) and 5B, 5C (proposed name). These genes present a nucleic tandem repeat organization. The choice of oligonucleotide probes was made to amplify the signal by hybridization of many small probes on the same mRNA molecules. A characteristic pattern of mucin gene expression was observed for each mucosa.