Blood lymphocytes from a boy with Bloom’s syndrome and a man with a mild form of xeroderma pigmentosum were examined for their response to UV-irradiation, as measured by the levels of induced unscheduled DNA synthesis, and to X-irradiation, as measured by chromosome aberrations. Although the individual with Bloom’s syndrome showed facial skin sensitivity to sunlight, his lymphocytes showed a normal response in unscheduled DNA synthesis. In contrast, cells from the individual with xeroderma pigmentosum showed a 50 % reduction in the amount of unscheduled DNA synthesis following UV exposure. Cytogenetic studies on PHA-stimulated Bloom’s syndrome lymphocytes revealed a typical increase in the incidence of aberrations. Many of the Bloom’s cells showed breakages located at the centromere and equal, symmetrical chromatid interchanges involving homologous chromosomes; these aberrations were not produced in control cells cultured in serum from the Bloom’s syndrome patient. In contrast, the frequency of spontaneous aberration in xeroderma pigmentosum lymphocytes was similar to that in controls. G1 cells from both patients were exposed to a range of X-ray doses, and it was shown that neither cell type differed from controls in the incidence of induced chromosome-type aberrations. The significance of these findings is