Mammary epithelium of adult C3H mice in contact with fetal salivary mesenchyme took morphogenetically a salivary gland-like pattern of growth and developed a hyperplastic nodular mass at the site of transplantation. When murine mammary tumor virus-bearing C3H mice were treated in this way, early development and high incidence of mammary cancer were observed from these nodules compared to unstimulated mammary glands of the same host. Since tumor development, the viral antigens in these hyperplastic nodules were examined by means of the immunofluorescence technique. As early as 2 weeks after mesenchyme transplantation, the antigens were observed in the ductular or alveolar structure of these nodules, while surrounding normal tissues were not stained. Positive fluorescence was also observed in the mammary tumors that developed from these stimulated nodules. These results strongly suggested the involvement of murine mammary tumor virus in the early development of mammary cancer induced by fetal mesenchyme transplantation.