Chromosome-type aberrations were found in 23 of the 43 exposed people and 5 of the 8 unexposed individuals (Table II). Proportionally, the largest number of persons with chromosome aberrations was found in the low exposure group with 84 per cent. The unexposed group was next with 62 per cent and this was followed by the high exposure group in which only 40 per cent of the people showed aberrations. In each of the two exposure groups one half of the aberrations consisted of two-hit events such as dicentric chromosomes, exchanges and a ring, the remainder being acentric fragments. The incidence of acentric fragments was unusually high in the unexposed group, but in contrast to the exposed people no two-hit aberrations were found among them. The difference between the exposed and unexposed groups with regard to the incidence of two-hit aberrations was found to be significant at the 1 per cent level (p<0.001). Aberrations ranged from one to four per person. Sixteen of the 23 exposed people (70%) with chromosome aberrations had more than one such aberration; two of the 5 unexposed (40%) had multiple aberrations. In most instances no more than one aberration was noted per cell. m4 There was no correlation between the occurrence of thyroid pathology and chromosome aberrations. Ten of the exposed people included in this group developed thyroid pathology since these examinations were made. Only four of them had shown chromosome aberrations earlier; three were children and one an adult at the time of exposure. No aberrations were found in the karyotypes that had been made from the 32 grossly intact cells from the four subjects showing relatively high aberration rates. Likewise, no abnormalities were seen in 50 cells from the bone marrow of one of the exposed subjects. Discussion The results of this study demonstrate that a small but significant number of chromosome aberrations has persisted in blood lymphocytes of some of the Marshallese people for ten years following exposure to whole-body gamma radiation from fallout. This conclusion rests on the finding of the large number of two-hit aberrations (dicentrics, exchanges and a ring chromosome) in the exposed people which constituted one half of all chromosome aberrations seen. This was in striking contrast to the findings in the unexposed people in whom no two-hit aberrations were found even though the overall number of chromosome aberrations in this group was about the same and even a fraction higher than that in the exposed people taken as one group. Since two-hit aberrations were seen only in the people who had whole-body gamma ray exposure and not in the unexposed group, we interpret this finding as a residual effect of radiation. We are unable to explain the greater incidence of chromosome aberrations in the group exposed to 70 rads as compared to the group that received 175 rads. We are also unable to account for the unusually high incidence of acentric fragments in the unexposed and their relative defecit in the exposed people. One might consider causative factors such as the low levels of internal radiation exposure from absorption of radionuclides such as 137¢5, 657n, Os, or exposure to such things as virus 139