To study self-renewal and differentiation of spermatogonial stem cells, we have transplanted undifferentiated testicular germ cells of the GFP transgenic mice into seminiferous tubules of mutant mice with male sterility, such as those dysfunctioned at Steel (Sl) locus encoding the c-kit ligand or Dominant white spotting (W) locus encoding the receptor c-kit. In the seminiferous tubules of Sl/Sl^d or Sl^17H/Sl^17H mice, transplanted donor germ cells proliferated and formed colonies of undifferentiated c-kit (−) spermatogonia, but were unable to differentiate further. However, these undifferentiated but proliferating spermatogonia, retransplanted into Sl (+) seminiferous tubules of W mutant, resumed differentiation, indicating that the transplanted donor germ cells contained spermatogonial stem cells and that stimulation of c-kit receptor by its ligand was necessary for maintenance of differentiated type A spermatogonia but not for proliferation of undifferentiated type A spermatogonia. Furthermore, we have demonstrated that their transplantation efficiency in the seminiferous tubules of Sl^17H/Sl^17H mice depended upon the stem cell niche on the basement membrane of the recipient seminiferous tubules and was increased by elimination of the endogenous spermatogonia of mutant mice from the niche by treating them with busulfan.