The functional role of airway mucin in the respiratory system is well recognized. The isolation of mucin cDNA clones, MUC genes, introduces new information regarding the structure of the mucin core protein; however, the nature of the authentic core protein of airway mucin is still unresolved. In this communication, the effects of vitamin A on the regulation of MUC2 gene expression in primary tracheobronchial epithelial (TBE) cells of human and nonhuman primates were examined. Vitamin A has been recognized as one of the most important nutrients in the regulation of airway mucous cell differentiation. The expression of the MUC2 gene has been demonstrated in both rat and human tracheal tissues. The monkey cDNA clone MT80 was isolated from a cDNA library derived from vitamin A-depleted cultures of monkey TBE cells using a synthetic oligonucleotide probe corresponding to the 69 nucleotides of a tandemly repeated sequence in human MUC2 cDNA. DNA sequencing revealed a similar tandemly repeated sequence, except that 72 oligonucleotide repeats were observed in the monkey cDNA clone. Using the MT80 cDNA as a probe, the expression of the MUC2 gene was studied in vitro. The corresponding MUC2 message level in primary cultures of monkey TBE cells was down-regulated by vitamin A. This result was consistently demonstrated in primary human and hamster TBE cultures. The down-regulation was both time- and dosage-dependent on vitamin A. A nuclear run-on assay demonstrated a decrease in the transcriptional rate of the MUC2 gene in nuclei isolated from vitamin A-treated cultures. These results suggest that MUC2 gene expression in TBE cells is transcriptionally down-regulated by vitamin A.