The objective of this program is the bioassay of new retinoid compounds by the tracheal organ culture assay. Hamster tracheal organ cultures provide an experimental assay system for determining whether new retinoids can alter epithelial cell differentiation. Under conditions of vitamin A deficiency, the tracheobronchial epithelium forms keratinized squamous metaplastic lesions, but in the presence of active retinoids, the process of keratinization is reversed toward columnar cilated and mucus-secreting cells similar to those observed in vitamin A-normal animals. Once data for a specific retinoid are obtained and evaluated, prompt reporting is made to the NCI Project Officer and contractor synthesizing the retinoid. Biological activity data have been provided on more than 150 retinoids from a total of over 10,000 hamster tracheas.