The role of bone marrow derived cells (BMD) within the thymus during the induction of immunological tolerance was investigated using transplantation of chimeric thymuses. Chimeric thymuses were constructed by reconstituting lethally irradiated BALB/C recipients with T-cell depleted C3H bone marrow (BM) cells. Two to three months post bone marrow transplantation the thymuses of these animals, in which the epithelium was of BALB/C origin and BMD cells of C3H origin (verified by immunoperoxidase staining), were transplanted into thymectomized, lethally irradiated BALB/C recipients which were reconstituted with T-cell depleted syngeneic (BALB/C) bone marrow cells. Induction of specific tolerance to the BMD cells (C3H origin) present in the chimeric thymus could be demonstrated in MLR at 3 to 6 months post chimeric thymus transplantation. It is concluded that bone marrow derived cells are able to induce immunological tolerance within the thymus.