Major leucocyte subpopulations were isolated from peripheral blood of healthy donors, and patients with chronic myeloid leukaemia (CML) and chronic lymphoid leukaemia (CLL). In vitro UV irradiation was performed at the wavelength of 257nm (UVC band). DNA double-stranded breaks (DNAdsbs) were detected immediately after UV-irradiation, by means of agarose gel electrophoresis. Cell viability was estimated after 18h in culture, as relative numbers of residual non-apoptotic cells. Evaluation of the dose-response curves revealed that normal CLL lymphoid cells showed only moderate damage after UV-irradiation, as assessed by DNAdsbs and cell viability criteria. However, normal granulocytes and myeloid blasts from CML patients expressed a sharp increase in DNAdsbs, even at lower doses of UV-radiation. UV-induced amplification of endogenous oxidative systems (e.g. NADPH-dependent oxidase) is suggested as a probable reason for enhanced DNA breakage and apoptosis in cells of the granulocytic lineage.