Lethally irradiated DBA/1 or (C57BL×DBA/1)F₁ mice were injected simultaneously with therapeutically effective doses of isologous bone marrow cells and syngeneic lymphocytes from intact mice (control) or from animals surviving different lengths of time after sublethal irradition. In the control the presence of lymphocytes in the mixed graft did not affect the survival rate of the recipients. Lymphocytes from mice surviving 6-12 months after irradiation in a dose of 600-700 R blocked the therapeutic effect of bone marrow (the killing effect). The intensity of the killing effect depended on the number of lymphocytes transplanted and the number of bone marrow cells in the graft. No killing effect was found if mice surviving 1 month after irradiation were used as donors of the lymphocytes. The results are regarded as evidence of autosensitization of the animal in the late stages after irradiation.