Transplantation of fetal mammary gland mesenchyme into mammary glands of 2-month-old syngeneic virgin mice resulted in focal re-enactment of events that normally occur probably during fetal and early postnatal development of the mammary gland. Portions of the recipient's mammary duct system in contact with the fetal mammary mesenchyme underwent branching and proliferation in a pattern resembling that of rudimentary mammary gland development. This process occurred in C3H mice regardless of whether or not the milk-transmitted mammary tumor virus (MTV-S) was present. In mice carrying MTV-S, mammary cancers of Types A and B appeared earlier and more frequently in the mammary glands that had received transplants of fetal mammary mesenchyme, compared with those in the glands that received no fetal mesenchyme. Some of the smaller cancers were shown to develop directly from portions of the mammary gland interacting with fetal mammary mesenchyme, without preformation of typical hyperplastic alveolar nodules. In C3H mice not carrying MTV-S, cancers did not appear in the similarly treated mammary glands. These facts suggest that non-hormonal and probably nonviral factors that stimulate focal proliferation in the mammary duct system resulting from transplantation of fetal mesenchymes eventually accelerate local development of mammary cancers.