Induction of mixed chimerism (i.e., coexistence of donor and recipient hematopoietic cells) through transplantation of allogeneic donor bone marrow under appropriate host conditioning, is one of the most reliable strategies to induce transplantation tolerance. Robust tolerance is evident in mixed chimeras as they permanently accept donor skin grafts while promptly rejecting third party grafts. Although historically, myeloablative and T-cell depleting regimens have been described, milder protocols involving costimulation blockade have recently been developed. The prototypical murine protocol described in this chapter, involves the use of CTLA4Ig and a monoclonal antibody-specific for CD154 (CD40L) for costimulation blockade, 3 Gy of nonmyeloablative total body irradiation and a conventional number of 20 × 10⁶ fully allogeneic bone marrow cells. Flow cytometry is used to determine levels of multilineage hematopoietic chimerism and deletion of donor-reactive CD4⁺ T cells. Tolerance is assessed in vivo by grafting of donor and third party skin.