Human and rabbit peripheral blood lymphocytes were irradiated in vitro with 1.5 Gy x-rays after exposure to 0.01 Gy of x-rays (dose rate: 0.01 Gy/min). The frequency of chromosome aberration in the cells pre-exposed to 0.01 Gy of x-rays at Go, G1, S and G2 phases of cell cycle respectively was markedly lower than that in the cells irradiated with 1.5 Gy alone (P less than 0.01). This adaptive response would last for 3 cell-cycles, beyond which the damage of the chromosome following the second large dose occurred with the same magnitude as that without a previous conditioning exposure. However, when the low dose was repeated after the third cell cycle the adaptive response could be revived. Meanwhile, it was found that the adaptive response could be also induced in vivo in the bone marrow cells and germ cells of mice by low dose irradiation. In addition, the effect of dose rate of the first irradiation on the induced adaptive response was also studied.