The lymphosarcoma could not be eliminated by total-body X irradiation with a supralethal dose. This is in accordance with the experience of other workers who treated transplantable tumors in animals by total-body irradiation. Injection of homologous or heterologous lymph-node cells in addition to bone marrow resulted in inhibition of tumor growth. However, the animals ultimately died either of lymphosarcoma or foreign bone-marrow reaction. It has been shown that antibodies produced by homologous or heterologous lymph-node cells probably play a major part in the inhibition of tumor growth. On the other hand, the debilitating effect of the foreign bone-marrow reaction cannot be excluded with certainty as a factor in the inhibition of tumor growth. The best results with respect to the survival of the animals were obtained after the injection of large numbers of isologous lymph-node cells. In the latter case, the inhibition of lymphosarcoma growth probably is a consequence of competitive proliferation of normal and irradiated neoplastic lymphoid cells in the irradiated host. In these experiments competitive proliferation could not be demonstrated after injection of large numbers of homologous lymph-node cells since this procedure is acutely lethal to mice.