Human bronchial epithelium has been maintained in organ culture in serum-supplemented medium for 4 months. After 4 to 6 weeks in culture, various changes in morphology were apparent. There was an increase in autophagic vacuoles in mucous, ciliated, and basal cells, a reduction in the height of the columnar cells, a decrease in the number of goblet mucous cells, and an increase in cells with small mucous granules. After 3 months in culture, the basal lamina was frequently covered by 2 or 3 layers of epithelial cells consisting of nonkeratinizing squamous cells with short microvilli and small mucous granules. Less frequently, keratinizing squamous cells were seen. Differentiated epithelium incorporated precursors into macromolecules in serum-free medium, supplemented with vitamin A, at 1 week of culture. These explants exhibited changed epithelium by 2 weeks, similar to that described for epithelium in serum-supplemented medium after 4 to 6 weeks.