Embryonic stem (ES) cells have been investigated in many animal models of severe injury and degenerative disease. However, few studies have examined the ability of ES cells to improve functional outcome following mammary gland injury. This study investigates the feasibility of implanting mouse ES cells labeled with enhanced green fluorescence protein in the developing mammary glands in order to acquire lineage-committed cells in mammary (mammary gland epithelial cell or luminal cell). Cells implanted in high numbers (5 × 10(6) cells per mammary gland) survived in the majority of the mice and nearly 38.4% of the surviving cells were CK18(+) at 15th week following the transplantation. Our results may provide a technique instrument on advanced therapy of breast diseases and the mammary regeneration after breast ablated partly.