The X-ray sensitivity of chromosomes from a Bloom's syndrome patient and a normal control was compared in G1 and G2. There was no significant difference in the number of aberrations induced by irradiation in G1. An increased sensitivity of the BS chromosomes was found in G2. The frequency of mitotic chiasmata in the BS cells was not increased by the G2-irradiation, even though the frequency of chromatid translocations was much increased. This provides further evidence for the fundamental difference of these two phenomena. Evidence from the types of aberrations induced suggests that there is no appreciably increased frequency of pairing of homologous chromosomes in the BS cells over that in normal cells.