The overall goal of this project is to determine the spectrum of phenotypic transformation possible with epithelial cells and to elucidate the basic biological events involved. The response of BALB/c murine mammary gland cells to SV40 and mouse mammary tumor virus (MMTV) is the system under study. Normal mammary epithelial cells have been transformed by SV40. Ultrastructural studies confirm the epithelial origin of the transformants. All the transformants express SV40 early proteins, exhibit anchorage-independent growth, grow to slightly higher saturation densities than normal cells, and multinucleate extensively in the presence of cytochalasin B. Three of the lines have formed tumors in mammary fatpads of BALB/c mice.