In previous experiments, murine myeloid leukemia virus has been found to induce only in some cases lymphatic leukemias. The leukemogenesis of the lymphatic types could be strongly inhibited by thymectomy. Leukemias occurring in thymectomized virus-infected animals were predominantly of the myeloid type. A combination of thymectomy and splenectomy was used because of the inhibitory effect of splenectomy in virus-induced myeloid leukemias. Injection of virus from selected lymphatic leukemias into double-operated animals reduced the incidence of leukemias to 13%. In the bone marrow of intact virus-infected double-operated mice, a strong multiplication of the virus has been demonstrated. Using isologous thymic cells in substitution experiments, the inhibitory effect of thymectomy on leukemogenesis could be greatly abolished (80%).