The in vitro pretreatment of phytohaemagglutinin stimulated human lymphocytes with tritiated thymidine or with low doses of X-rays induces a response that makes these cells less susceptible to further genetic damage induced by subsequent high doses of radiation. This phenomenon has been called 'Adaptive Response' because it is similar to the one described in E. coli. In the present study, we describe that lymphocytes irradiated in vitro at 2 Gy, from individuals occupationally exposed to X and gamma rays, show lower frequencies of dicentrics than those from non-occupationally exposed individuals irradiated in the same conditions. Our results could indicate that an adaptive response can also be induced in lymphocytes in vivo by very low occupational doses of radiation, and that for biological dosimetry purposes it is necessary to know the individual's history of exposure to mutagens.