We report on 2 cell lines (designated OVAK-I and OVAK-II) established from an adenosarcoma, a rare variant of a malignant Mullerian mixed tumor, of the ovary. OVAK-I was not tumorigenic in nude mice and showed no responsiveness to transforming growth factor (TGF)-beta1, whereas OVAK-II was tumorigenic and had various biological properties. Light- and electron-microscopic studies showed that the morphology of the cells was converted from an epithelial to a mesenchymal form by TGF-betaI. Furthermore, the protein and mRNA expressions of the epithelial marker carcinoembryonic antigen and the mesenchymal marker vimentin were down- and up-regulated, respectively, by TGF-beta1. These findings suggested that TGF-beta1 plays a role in epithelial-mesenchymal transformation and support the single-stem-cell theory which explains the origin of malignant Mullerian mixed tumors: that the epithelial and mesenchymal components of these tumors are of common stem-cell origin.