A chromosomal adaptive response to ,60,Co γ-rays in human lymphocytes was observed with pretreated doses ranged from 1.0 to 20 cGy, with 1.0 cGy gave optimal reduction in chromatid breaks. A 0.5 cGy dose which in itself did not induce an adaptive response, did so when given twice within the same cell cycle, and the magnitude of accumulative effect is the strongest when there was an interval of 6 h between the two adaptive doses and between the second adaptive dose and a challenge dose. Reductions equivalent in effect to a single 1.0 cGy dose were seen when a 0.5 cGy dose was given twice. Delivering two 1.0 cGy doses had no greater effect than did a single dose of 1.0 cGy.