The mouse is frequently used as an experimental animal in studies on pulmonary carcinogenesis. Potential stem cells for primary pulmonary neoplasms in murine distal airways, which have received considerable interest 113, are the type I1 alveolar epithelial cell and the nonciliated columnar (Clara) cell. How-ever, despite the relevance of the issue, there is still no consensus about the question as to which of these cell types is the cell of origin of primary pulmonary neoplasms in mice. As recently reported [I], this evaluation may be facilitated by following a new approach, namely comparison of specific antigenic and ultrastructural features of tumor cells on the one hand and type I1 cells and nonciliated columnar (Clara) cells in early and late embryonic lungs on the other hand. Two latter two cell types first appear in mouse embryos with a developmental age (da)(basis: embryonic weight [2]) of about 14.2 days [2, 31. This means that these cell types are detectable 3 to 4 days earlier than generally has been assumed in the literature [4, 51. The purpose of this review is to summarize the knowledge of mouse lung development. As explained later, my research in the mouse [2, 4-61 has led to a novel view on mammalian lung development [5] and adult lung structure [6]. Furthermore, I will give a survey of the immunocytochemical and ultrastructural features of type I1 cells and nonciliated columnar (Clara) cells as found in our studies of embryonic and adult mouse lungs [l-91.