After gently trypsinization, the pseudostratified columnar Müllerian epithelium that lines the uterine cervix of newborn mice could be separated from the enclosing stromal tissue Pure epithelial tubes explanted in vitro and were allowed to grow in a standard medium for 3-4 days forming a confluent colony of rather closely-fitting cells. The cell sheet was studied by a preparatory technique that allows examination of a large number of cells with preserved intercellular spatial orientation. Attempts were made to identify cultured cells according to the morphology of cell types in the cervicovaginal epithelium in vivo. Electron micrographs revealed that, close to the explant, the cultured cell sheet exhibited several features similar to the Müllerian epithelium in vivo. Outside these central areas of the colony was a broad transitional zone consisting of thin platelike cells distinguished by an abundance of microfilaments. At the periphery of the colonies, bulky cells possessing microvilli and a vacuolated cytoplasm tended to overlpa adjoining platelike cells. These bulky cells had a morphology resembling that of the superficial cells seen in the upper vagina and common cervical canal of immature and diestrous animals. The epithelial development in the cultures apparently simulated the transformation iatified epithelium resembling that of the uppermost vagina and common cervical canal of immature animals. Judged by morphological and cytochemical criteria, the Müllerian cells in the out-growth obviously had many changed features. It thus seems questionable wheter the cells grown in vitro are comparable with the corresponding cells in vivo when used for experiments requiring the corresponding of the culture environment.