Development of nontoxic and nonmyeloablative regimens for allogeneic hematopoietic stem-cell transplantation will decrease transplantation-related mortality caused by regimen-related toxic effects. In pursuit of this goal, a dog model of stable mixed hematopoietic chimerism was established in which leukocyte-antigen–identical litter mates are given sublethal total-body irradiation (2 Gy) before stem-cell transplantation and immunosuppression with mycophenolate mofetil and cyclosporine afterward. In the current study, we examined whether donor lymphocyte infusion (DLI) could be used as adoptive immunotherapy to convert mixed to complete donor chimerism. First, 8 mixed chimeras were given unmodified DLI between day 36 and day 414 after stem-cell transplantation. After a 10- to 47-week follow-up period, there were no significant changes in the percentage of donor engraftment. Next, we immunized the donor to the minor histocompatibility antigens (mHA) of the recipient by means of repeated skin grafting. Lymphocytes from the mHA-sensitized donor were infused between day 201 and day 651 after transplantation. All 8 recipients of mHA-sensitized DLI had conversion to greater than 98% donor chimerism within 2 to 12 weeks of the infusion. Complications from mHA-sensitized DLI included graft-versus-host disease in 2 dogs and marrow aplasia in 1. These results showed that the low-dose transplant regimen establishes immune tolerance, and mHA-sensitized DLI is required to break tolerance, thereby converting mixed to complete donor chimerism. We propose that mixed chimerism established after nonmyeloablative allogeneic stem-cell transplantation provides a platform for adoptive immunotherapy that has clinical potential in the treatment of patients with malignant diseases.