ln the radiosensitive spleen, bone marrow, and thymus, total-body, lethal x irradiation of the mouse produces a rapid increase in acid deoxyribonuclease activity per average cell. At the same time, a decrease in nn inhibitor of serum alkaline deoxyribonuclease is demonstrable in spleen homogenates. Post-irradiation infusion of homologous bone marrow is followed by the return of the enzyme and inhibitor to normal activity as the tissues are repopulated by normal cells. The relatively radioresistant liver, which undergoes much less loss of cellularity, shows correspondingly small enzymatic change not reversible by bone marrow infusion. The radioresistant kidney shows no significant changes in either cellularity or the enz~e systems studied. (auth)