Single-cell-cloned cell lines intermediate in morphology between the cuboidal epithelial and fully elongated myoepithelial-like cells have been isolated from the single-cell-cloned epithelial stem cell lines Rama 25 and Rama 37 originally obtained from dimethylbenz(a)anthracene-induced mammary tumors from Sprague-Dawley and Wistar-Furth rats, respectively. These are designated Rama 25-I1, Rama 25-I2, Rama 25-I4 (Sprague-Dawley) and Rama 50-55, Rama 59, and Rama 60 (Wistar-Furth), respectively. When growing as tumors in nude mice or syngeneic Wistar-Furth rats, respectively, many of the newly cloned cell lines give rise to spindle and giant, multinucleated cells which stain immunocytochemically with antisera to myoglobin and myosin and contain longitudinal fibrils, some of which contain phosphotungstic acid-hematoxylin-staining cross-striations. Ultrastructural analysis demonstrates the presence of A-, I-, and H-bands and Z-discs and the hexagonal arrangement of thick and thin filaments characteristic of skeletal muscle. Similar results are obtained with selected cloned cell lines growing on floating collagen gels in vitro. Thus, a developmentally committed mammary epithelial cell can give rise, under suitable conditions, to a well-differentiated mesenchymal lineage, that of skeletal muscle. It is suggested that such cells may be responsible for the generation of the well-differentiated mesenchymal elements seen in the mixed (epithelial and myoepithelial) tumors of glandular origin.