Peripheral blood lymphocytes isolated from a group of 41 temporary nuclear plant workers receiving doses ranging from 0 to 10 mSv showed no increase in the baseline micronuclei frequencies as compared to the control values before the in vivo dose (Thierens et al., 2002). After an in vitro challenging dose of 3.5 Gy 60Co g-rays, given either at a high-dose rate (1 Gy/min) or a low-dose rate (4 mGy/min), the number of micronuclei was statistically lower for the exposed persons as compared to the non-exposed persons. Interestingly, the level of adaptation was elevated if the challenging dose was given at a low-dose rate.