Using the model of chemically induced leukemogenesis of mice by N-nitroso-N-methyl urea (NMU) studies were done to find out the primary point of attack of the chosen carcinogen on cellular level. After application of 14C-NMU (XVII X AKR) F1 hybrid mice were killed at different times, and the 14C activity was determined in various organs by scintillation counting and autoradiography. Contrary to expectations, the bone marrow showed a significantly higher activity than the thymus, which is supposed to be the target organ for lymphatic leukemogenesis. The specificity of the enrichment of 14C activity in bone marrow could be assured by comparative proliferation studies. The autoradiographic results favor the lymphatic cells of bone marrow as target for NMU. The target cell problem is discussed in respect to thymectomy and recent results on nude mice. With high probability the thymus is not essential for lymphatic leukemogenesis and, consequently, is not the target organ.