Lethally irradiated DBA/l mice or (C57BlXDBA/1) F₁ hybrid mice were injected with therapeutically effective doses of isologous bone marrow cells; simultaneously syngeneic lymph node cells from either intact (control) animals or mice that survived after sublethal irradiation were transplanted. The viability of the recipients was not affected by the presence of lymphoid cells in the mixed transplant. In contrast, the beneficial action of the bone marrow cells was abolished (killing-effect) by the lymphoid cells from mice sacrificed 6 to 12 mth after the irradiation (600-700 rad). The manifestation of the killing-effect depended on the number of the transplanted lymphoid cells and on the dose of the bone marrow cells in the transplant. The killing-effect was not revealed when the lymphoid cells were obtained from the donors on the 30th day after irradiation. The results suggest the autosensitization of the organism at the late postirradiation periods.