This study was aimed to explore the effects and mechanisms of transplantation tolerance induced by "TBI + cyclophosphamide (CTX)" regimen combined with intra-bone marrow injection of allogenic BMCs. On day 0 C57BL/6 (H-2(b), B6) mice received sublethal dose of total body irradiation (TBI) ((60)Co gamma-ray) followed by intrabone marrow-bone marrow transplantation (IBM-BMT) of 3 x 10(7) cells/30 microl BMCs from BALB/c (H-2(d)) mice. The recipient mice were given CTX intraperitoneally 2 days after IBM-BMT. On day 7 skin grafting was performed and the skin survival was observed. The tolerance mechanism was investigated by mixed lymphocyte reaction (MLR), IL-2 reverse test, adoptive transfer assay in vitro. The results showed that the mean survival time (MST) of skin allografts in group treated with TBI + CTX + BMT was significantly longer, compared with that of other groups (P < 0.01). On day 90 after IBM-BMT, the phenotypic character of the recipient mice (black color) began to convert to that of the donor mice (white color). The MLR demonstrated that the immune responses of recipient mice were donor-specific tolerance. Suppressive activity in the spleen cells of tolerant B6 mice was observed in adoptive transfer assay in vitro. IL-2 reversal and the phenotypic conversion showed that the tolerance mechanisms were involved in clonal anergy and the development of chimerism. It is concluded that the nonmyeloablative regimen combined with IBM-BMT can induce a long-term tolerance, and the multiple mechanisms including clonal anergy, suppressor cells and chimerism were involved in transplantation immune tolerance.