The survival of skin grafts from B6.C-H-2bm1 (bm1; Kbm1,IAb,IE−,Db) mice was prolonged when C57BL/6 CrSlc (B6; H-2b) mice were inoculated intravenously with 9±107 spleen cells (SC) plus 3±107 bone marrow cells from bm1 mice 14 days prior to skin grafting. When B6 mice were inoculated i.v. with bm1 cells and treated intraperitoneally with 200 mg/kg cyclophosphamide (CP) 2 days later, the survival of bm1 skin grafts was not prolonged at all, suggesting antagonism between the veto cell-mediated enhancement by donor cells and CP-induced enhancement. In order to deplete the veto cells from the tolerogen, SC plus BMC were treated with anti-Thy1.2 mAb + C± or CP. The survival of the bm1 skin grafts was not prolonged at all in B6 mice inoculated with Thy1.2-treated bm1 or bm1 cells from CP-treated donors. When B6 mice were inoculated with Thy 1.2-treated or CP-treated bm1 cells and followed by CP treatment, however, the survival of bm1 skin grafts was prolonged moderately. These results strongly suggested that the unresponsiveness induced with donor-derived veto cells prevents the tolerance induction to class I alloantigens (H-2Kbm1) by our protocol of CP-induced tolerance. Furthermore, in B6 mice injected with anti-Thy1.2 mAb on day −1 and bm1 cells on day 0, the survival of bm1 skin grafts was not prolonged at all. However, skin graft tolerance to bm1 antigens was induced when B6 mice were injected with anti-Thy 1.2 mAb on day −1 and bm1 cells on day 0 followed by CP on day 2. These results may also be explained by the depletion of donor-derived veto cells in the recipient mice.