Mice were abdominally injected with a adaptive dose of 3.7× 10,2,—3.7×10,5, Bq/gbw HTO, and then exposed to a challenge dose of 1.5 Gy of ,60,Co γ-rays. In bone marrow cells that received both the adaptive and challenge doses, the chromatid breaks are lower than expected on the basis of additivity of the effects of the individual treatment. The adaptive response induced with 3.7×10,3,Bq/gbw HTO is the most remarkable, but at 3.7×10,5,Bq/gbw the adaptive response seems to be disappear. The adaptive response can be observed by exposing to 1.5 Gy γ-rays from 1 to 5 days after injection of 3.7×10,3,Bq/gbw HTO, which is the most obvious at the 5 th day, to reduce chromatid breaks to 50% of expected, but at the 7 th day to equat to expected. The frequency of chromatid braeks is gradually reduced with time after challenge dose, the maximum index number of adaptive response is 0.50 and appears at 24 hr after challenge dose.