The adaptive response was studied in preimplantation embryos and spleen lymphocytes of a mouse inbred strain and in peripheral lymphocytes of three human donors, using chromosomal aberrations as the endpoint. Embryos were adapted to 0.05 Gy X-ray 50 h post-conception either in vitro or in vivo and challenged 6 h later. Chromosome aberrations of the 8----16 cell stage mitoses were scored. No adaptive response was seen in the embryos. Of 14 female mice studied, an adaptive response was seen in spleen lymphocytes of only one mouse. However, because variable chromosomal aberration levels were observed in lymphocytes of different donors, it is concluded that the adaptive response detected was merely a result of this heterogeneity. In human peripheral lymphocytes an adaptive response was seen in all three donors. It is speculated that the inbred mouse strain used is deficient in the adaptive response.