Long-term culture of the middle ear epithelium of guinea pigs was carried out on reconstituted floating collagen matrix. Fibroblasts established from the abdominal skin dermis of allogenic animals were used to reorganize hydrated collagen gels into a dermal-like matrix. The explants placed on the surface of these matrices were composed of pseudostratified columnar cells with polygonal flat outgrowth sheets, which could be maintained for up to one month. In contrast, with simple hydrated collagen gel, no lysis of the substrate was observed regarding this reorganized collagen gel. In order to examine the growth pattern of these culture cells, 5-bromodeoxyuridine (BrdU) was added to the medium during the entire culture period. The specimens were immunohistologically stained using monoclonal antibodies. Noticeable changes were observed in the marginal portion of the explant where the cell shape changed from cuboidal to squamous. In this transitional area, most cells showed positive staining and consequently high proliferative activity. In the central area of the explants, however, the number of positive cells decreased and the main observation was a few labeled basal cell nuclei. It was suggested that the same regenerative process which usually occurs in normal respiratory epithelia after mechanical injury or other insults, was replayed in this culture system.