[en] Human lymphocytes pre-exposed to low doses of ionizing radiation show an adaptive response, which make these cells less sensitive to subsequent higher exposures. To verify the hypothesis that a similar phenomenon can be induced by occupationally received doses of ionizing radiation, the cytogenetic responses of fourteen nuclear power plant workers to γ-irradiation was tested. Peripheral blood lymphocytes from nuclear power plant workers were tested for their sensitivity to induction of unstable chromosome aberrations by a subsequent exposure to a high dose of γ-irradiation (2Gy). The results show that lymphocytes of occupationally long-term exposed nuclear power plant workers are not sensitized and protected to subsequent high dose radiation with unstable chromosome aberrations induction