In this study the specific immunological tolerance of donor cells towards the host tissue antigens in allogeneic mouse irradiation chimeras was investigated. It has been shown that transfer of bone marrow cells from these chimeras into irradiated mice of the original bone marrow-donor strain results in a loss of tolerance. However, transfer of spleen cells or bone marrow cells together with lymph node cells results in a prolonged state of tolerance towards the tissue antigens of the first host, as could be demonstrated with skin-grafts. These transfer experiments make it probable that for the maintenance of the tolerance within the lymphatic cell population no antigens are necessary. Mice inoculated with a tolerant lymphatic cell population gradually lose their tolerant state, the loss being ascribed to a gradual replacement of the tolerant population by non-tolerant cells originating from the transferred bone marrow. Thymectomy was therefore used as a method of preventing the transferred bone marrow from replacing the transferred tolerant population of lymphatic cells. The results with thymectomized hosts showed, that those mice which received only chimera bone marrow had a general nonreactivity to allogeneic tissue antigens, whereas the mice which received bone marrow and lymphatic cells from the tolerant chimeras showed a lasting and specific nonreactivity (tolerance) to the antigens of the first host. Thymocytes were only effective in maintaining the tolerant state when they were transplanted into thymectomized hosts.