The induction of an adaptive response of human lymphocytes from three donors exposed to 0.01 or 0.05 Gy of X-rays at 32 h of culture and to 1.5 Gy at 48 h was studied. There was no evidence for a significant reduction of the incidences of chromosomal aberrations in cultures pre-treated with low doses and subsequently exposed to the higher dose compared with cultures exposed to the high dose alone.