In adult animals, the type II pneumocyte is progenitor of both the type I and type II alveolar epithelium. In primary culture, however, the fate of this cell is uncertain. Type II cells in culture lose their differentiated properties and eventually resemble type I cells, but a lack of specific markers has complicated the characterization of the phenotype acquired in vitro. Furthermore, limited proliferation of these cells in vitro has precluded definition of the relationship between type II cell proliferation and differentiation. Recent work in this laboratory has involved the correlation of flow cytometric cell cycle analysis with phenotype markers. Initial results indicate that isolation of type II cells induces cell cycle block similar to that sustained by other cell types in response to stress. In addition, preliminary evaluation of phenotype suggests that traditional markers become ambiguous beyond the 1st day of primary culture. These results raise concern related to the interpretation of experiments conducted in vitro. This report discusses the implications of these findings and directions for future work.