A priming X-ray exposure to 0.05—0.10 Gy or 0.3—0.5 Gy imparts radioresistance (decrease in bone marrow death rate after high-dose X-rays) in mice 2—2.5 months or 9—17 days postexposure, respectively. This radiation-adaptive response in whole animals differs from that in cells observed several hours to several days (at most) later. An adaptive response was observed in ICR and C57BL/6 strains of mice, but not in BALB/c and C3H strains. The biological mechanisms of an acquired radioresistance induced by a priming exposure to 0.45 Gy in C57BL/6 mice have been studied. The recovery of blood-forming stem cells, determined as endogenous spleen colonies, was markedly stimulated by priming irradiation. The reduction in bone marrow death seems chiefly due to the stimulated recovery of blood-forming stem cells. But mice pretreated at this priming dose did not show a marked recovery of peripheral blood cell counts after challenging irradiation with mid- or sublethal X-ray doses (a significant, albeit slight, increase in the blood cell counts in the preirradiated group was observed after a lower challenging exposure to 5.0 Gy). The adaptive response depends on p53, as observed in cell experiments. The stem cells might produce an unknown factor(s) that contributes to acquired radioresistance. In a preliminary experiment, the life span of C57BL/6 mice after ascertaining their 30-day survival rates was not shortened by preirradiation.