Indications of an adaptive response in C57BL mice pre-exposed in vivo to low doses of ionizing radiation.

C57BL/6 mice were whole-body irradiated with 5 cGy/day ('adapting dose') on 4 consecutive days and their spleens removed on day 1, 3, 7, 12, 19 or 26 after the last irradiation. In vitro UV-light-induced unscheduled DNA synthesis (UDS) and mitomycin C (MMC)-induced sister-chromatid exchanges (SCEs) were scored in lymphocytes (UV-light and MMC being the 'challenging agents'), yielding higher UDS values and lower frequencies of induced SCEs than cells of non-adapted animals. On day 12 this effect could only be seen in half, on days 19 and 26 in none of the performed experiments. The results support those published by Tuschl et al. (1980, 1983) and Liu et al. (1987), showing that it is possible to induce the adaptive response in vivo.