To test the effect of chronic pre-exposure of low dose ,60,Co γ-rays on the induction of the adaptive response in vivo, four groups of mice ,were, whole-body irradiated with different doses of 9.3, 18.2, 37.2, and 72.9 mGy at 7.8, 15.1, 31.0, and 61.0×10,-3, mGy/min respectively, and then irradiated with 1.5 Gy γ-rays 3 hours after the termination of pre-exposure. Mice bone marrow chromosome preparations were made 24 hours later. The results showed that mouse bone marrow cells pretreated with 18.2, 37.2 and 72.0 mGy but not 9.3 mGy,60,Co γ-rays yielded significant lower frequencies of chromatid aberration and acentric fragment induced by subsequent high dose exposure than expected, but did not yield significant lower freruencies of dicentric and ring. However significant reduction of aberrant cell frequencies induced by subsequent high does exposure were observed in all the four different pretreated dose groups. The magnitude of the reductions of total chromosnme aberration and aberrant cell frequencies increased with the increasing of the pretreated doses.