We describe a method for collection and transplantation of mammary anlagen from mouse embryos that survive past embryonic day 13. The mammary glands are dissected from the embryos, cultured for a short period of time and transplanted into the cleared fat pad of 3 week old hosts. In this orthotopic site the mammary epithelium is exposed to ideal conditions for development: the pubescent host provides an authentic hormonal milieu for ductal growth and the necessary growth factors are secreted by the fat pad of the host. The transplanted glands will fill the fat pad within 6 to 8 weeks after which period the transplants can be harvested to evaluate development in virgins or the hosts can be mated to study development during pregnancy