Mice of the AKR strain were thymectomized at the following ages: 3, 35, 150, and 180 days after birth. Thymectomy was, in all stages tested, effective in preventing leukemia development. The incidence did not differ significantly and was below 15% among all the groups of mice thymectomized at varying ages, indicating that thymectomy at the preleukemic stage prevents leukemia development. The role of the thymus in connection with thymic humoral factor in leukemogenesis of AKR mice is briefly discussed.