Repeated intravenous injections of 7,12-dimethylbenz(a)anthracene (abbr. DMBA), four times weekly, induced several kinds of tumors in 26 mice (83.9%) out of 31 ddO mice, including 13 leukemias and 6 ovarian tumors. All leukemias were of lymphatic type, and 8 cases out of 13 leukemias were of thymus type. The thymus was involved by leukemic cells in 12 cases. All ovarian tumors were diagnosed as granulosa cell tumors histologically. During the treatment of DMBA, two types of changes in organs and blood were observed: one was a striking decrease of weight or organs or number of cells followed by prompt recovery as found in the spleen, red blood cells, and granulocytes in the peripheral blood; the other was moderate but long-standing decrease with little recovery as found in the thymus, ovaries, uterus, and lymphocytes in the peripheral blood. Unlike DMBA, the weight of the thymus was strikingly decreased by predonisolone, one of corticosteroids, whereas reactions of the spleen and the lymph nodes were not remarkable. Autoradiographical studies revealed a severe reduction followed by a marked rebound of labeling index in the red pulp of the spleen, but not in the white pulps of the spleen and cortex of the thymus. From these observations, it is concluded that a mild but long standing injury was noticed in the tissues which were the main sites of the tumorigenesis in mice by DMBA, and a severe but transient injury may not be related to induction of tumors in mice by DMBA.