Human lymphocytes exposed to very low doses of DNA damaging agents may become less sensitive to subsequent higher doses of the DNA damaging agent. This phenomenon is called the adaptive response.The aim of this project was to study the significance of DNA double-strand break (DSB) repair in the adaptive response of human lymphocytes exposed to ionising radiation. Human lymphocytes isolated from whole blood and stimulated with phytohemagglutynin (PHA) were irradiated with adaptive dose (5 cGy of X-rays) and then challenge dose of 2 Gy in case of micronuclei assay and 10 Gy in case of comet assay. The frequency of micronuclei in adapted lymphocytes was about 30% lower than that expected for an additive effect of both, adaptive and challenge doses, applied separately. Estimation of DSBs was carried out with the use of single gel electrophoresis assay (comet assay) in neutral pH and to complement these results with pulse-field gel electrophoresis (PFGE) pulse-field. The use of both PFGE and comet assay allowed us to suggest that lower damage revealed in the adapted lymphocytes at the chromosomal level was unrelated to initial level of DSBs in DNA. The differences between kinetics of DNA repair in the adapted and non-adapted lymphocytes were not significant.