en Radiation Biology Radiation Biology تحقيق بديع Original Research <p><strong>Background </strong>: There are growing evidences for chromosomal radioadaptive response in human lymphocytes . Highly variable inter- and intra-individual responses have been reported. Some individuals are non-responders and even in some donors the frequency of chromatid aberrations induced by a challenge dose increases by pre-exposure to an adapting dose. It has been proposed that the lack of radioadaptive response is due to transient physiological factors. </p><p><strong>Materials and Methods </strong>: We found a young healthy donor who exhibited no radioadaptive response in our initial experiments. After a common adapting dose, the donor occasionally showed a highly increased susceptibility to subsequent high-dose irradiation. To assess whether the lack of radioadaptive response and the induction of a synergistic effect are transient responses , we have performed a 3-year follow-up study employing micronuclei in binucleated cells besides chromatid aberrations as biological endpoints. To eliminate the effect of the cell cycle on intrinsic radiosensitivity of a cell, we used the multiple-fixation regimen for analysis of chromosomal aberrations. </p><p><strong>Results </strong>: This donor showed no adaptive response in any experiment. </p><p><strong>Conclusion </strong>: Considering the consistent non-responsiveness observed throughout our serial experiments, it may be concluded that the lack of radioadaptive response is not attributed to some transient physiological factors but rather to permanent constitutional traits. <i>Iran. J. Radiat. Res., 2003 1(1): 55 - 61. </i></p> Radioadaptive response, chromosomal aberrations, human lymphocytes, synergistic effect, CB-micronucleus assay. 55 61 http://ijrr.com/browse.php?a_code=A-10-1-9&slc_lang=en&sid=1 S.M.J. Mortazavi smj_mortazavi@yahoo.com 7900319475328460026 7900319475328460026 Yes T. Ikushima 7900319475328460027 7900319475328460027 No H. Mozdarani 7900319475328460028 7900319475328460028 No