Testicular cell transplantation has generally been performed by using immune-deficient recipient mice to investigate the biology of spermatogonial stem cells (SSCs), the production of transgenic animals, and restoration of fertility. Recently, we demonstrated that rat spermatogenesis can occur in the seminiferous tubules of immune-competent recipient mice via pretreatment with busulfan (Myleran, 1, 4-butanediol methanesulfonate, 40 mg/kg) after transplantation of rat SSCs. However, considering the immunosuppressive effect of busulfan, there is a possibility that busulfan itself causes immune suppression in immune-competent recipient mice. The aim of this study was to determine the effects of busulfan on the immune system and spermatogenesis in immune-competent recipient mice. The results showed that at 60 days after busulfan treatment, just the same time as the transplantation, the recovery could be seen in the immune system including cell counts and functions of T and B lymphocytes in the spleen, but the spermatogenesis was more compromised. This study demonstrated that after busulfan pretreatment the immune system in immune-competent recipient mice had recovered by the time that rat spermatogenesis could occur in the murine testis. It became clear that xenogenic spermatogenesis can be tolerated in seminiferous tubules in the testes of immune-competent mice.