Repopulation of rat tracheas of human tracheobronchial epithelial cells obtained from intermediate autopsies was achieved by introducing into de-epithelialized rat tracheas either pieces of donor tissue containing respiratory mucosa or epithelial cells produced by an in vitro amplification of these cells. After tracheas were sealed, and transplanted into the subcutaneous tissues of nude mice, a newly formed epithelium migrated over the denuded luminal surface. During this process, regenerative epidermoid metaplasias, consisting of the growth of thin stratified epithelium with keratinization but without atypia was observed. Four weeks after xenotransplantation, most of the luminal surface was covered by columnar epithelium with occasional patches of epidermoid metaplasia. When this epithelium was exposed to 7,12-dimethylbenzo[a]anthracene (DMBA) a thick epidermoid metaplasia with mild to moderate atypia was observed. This type of epithelium is seen one to three months after insertion of the DMBA-containing pellets into the tracheal lumen. Immunohistochemical staining with antikeratin monoclonal antibodies AE1 and AE3 revealed increased immunostaining in both regenerative and DMBA-induced metaplasias compared with that of untreated normal mucociliary epithelium. Although no differences between the two types of metaplasias were detected with AE1 and AE3, the use of involucrin immunostain showed important differences. Normal respiratory epithelium did not contain involucrin, but this protein was seen in the surface layer of regenerative epidermoid metaplasias. In DMBA-induced metaplasias, involucrin was found not only in the superficial cells but was also present in numerous suprabasal cells. The hyperplastic nature of these carcinogen-induced lesions, together with the presence of cellular atypia and an altered involucrin distribution pattern, suggest a preneoplastic state.