10) but is capable of potentiating the leukemogenic effect of other agents (2, 8). Injury to two tissues, thymus and bone marrow, has been shown to be an important feature of leukemogenesis by ionizing radiation (6). Thymic injury is manifested histologically as well as by decrease in thymic weight. Bone marrow injury is expressed functionally as a de crease in the capacity of bone marrow cells to promote rapid thymic regeneration. Is injury to these two tissues also involved in urethan leukemogenesis? Are there age dependent differences in urethan-induced injury which might explain the diminished leukemogenic potency of this agent in adult mice? Experiments relating to these questions are the subject of this report.