Vitamin A (retinol) is required for the normal mucociliary differentiation of respiratory epithelium. A depletion of vitamin A promotes squamous cell metaplasia. To understand how vitamin A suppresses squamous cell differentiation, the expression of a squamous cell differentiation marker, the small proline-rich protein gene (spr1), was studied in cultured monkey tracheobronchial epithelial (TBE) cells. The expression of the spr1 gene was inhibited about 40 fold by retinol. The mRNA levels of the spr1 gene started to decline within 6 h of retinol treatment and reached a minimum level after 7 days. The inhibition by retinol was concentration dependent and did not require concurrent protein synthesis. The inhibition of the spr1 mRNA by retinol was not due to a decrease in the transcription rate of its gene but due to a decrease in its stability, as determined by nuclear run-on assays and mRNA half-life measurement, respectively. This result was further supported by a DNA transfection study using a chimeric construct containing the spr1 promoter region and the chloramphenicol acetyltransferase (CAT) reporter gene. The CAT activity in transfected cells was not inhibited by retinol. These results suggest that spr1 gene expression is posttranscriptionally down-regulated by retinol.