In experiments with mice of a high leukemia strain, (Ak), bred over a period of years, it has been shown that the removal of the thymus has the effect of prolonging life and of decreasing the incidence of leukemia from 80.8 per cent to 9.9 per cent. Thymectomy was performed at about six weeks of age on 219 mice. The controls were nonthymectomized members of the same strain kept in the laboratory, until natural death during the course of the experiments. The leukemias in normal Ak mice dying at 7-12 months are predominantly lymphoid. However, in thymectomized mice, leukemias occurred at a later age and were of unusual types. These observations revealed that Ak mice are susceptible to leukemias other than lymphoid, which are masked in normal mice by early death from lymphoid leukemia. The thymectomized mice frequently had osteomas of the long bones. The data are best interpreted by assuming that the thymus contains potentially malignant cells, and by removal, the animal is spared from leukemia, and thus life is prolonged. Hence, the mice reach an age when other neoplasms with longer periods of latency become manifest. While these observations are not directly applicable to man, it is known that many neoplasms arise from organs which have survived their usefulness. However, such operations obviously are not feasible in man.