Primary cultures of middle ear epithelium from the guinea pig were successfully established on type I collagen coated dishes. To characterize cellular outgrowth, antibodies to the proliferating cell nuclear antigen were used as a marker for spreading cells in the S phase of the cell cycle. A number of migrating epithelial cells positively stained for proliferating cell nuclear antigen after 7 and 14 days in culture. Confocal laser scanning microscopy was used to evaluate the localization pattern of this antigen, and the fluorescence intensity was quantified in different areas of the migrating epithelial sheet after various times in culture. Two distinct areas proved to be major sites of proliferating cell nuclear antigen expression. One was at the edge of the tissue explants from which multilayered epithelial cells had begun to migrate. The other was along the margin of the outgrowth, where the cells often had elongated shapes and were aligned in rows. The cells in both areas were identified as nonciliated cells; ciliated cells in the outgrowth showed little staining. We hypothesized that the outgrowth cells in this experiment might be identical to the migrating cells usually observed in renewing epithelia after injury. This model may provide a simple and reproducible method of evaluating the regenerative ability of the middle ear epithelium.