To study the relationship between the DNA content of human chromosomes and their involvement in radiation-induced structural chromosome aberrations, the frequencies of translocations and dicentrics induced in human lymphocytes after in vitro exposure to 2Gy of x-tay were analysed by fluorescence in situ hybridization(FISH). Single whole chromosome probes(WCP), specific for chromosomes 1, 2, 4, 7, 8, 9, and 21 and triple combination of probes tot chromosome 1, 2 & 4 were used separately. A significant deviation of the frequency of symmetrical translocation and dicentrics from a DNA-proportional distribution was observed. Chromosomes 2, 7, 8, 9 and 21 were less frequently involved in the formation of symmetrical translocations and dicentrics than expected, whereas chromosomes 1 and 4 were more frequently involved. Chromosome 2 and 4 showed a higher frequency of acentric fragments. When triple combination probe for chromosome 1, 2 & 4 was used, no differences were found between the observed and expected frequency of exchange type aberrations. The results showed that the frequency of radiation-induced chromosome aberrations was not proportional to DNA contents, suggesting the difference in the susceptibility to specific aberrations among individual chromosomes. The results also indicated that the FISH technique with combination of probes for chromosome 1, 2 & 4 was useful for radiation biodosimetry.