1240 aged further, but the Marshallese who had greater damage had no uptake, and so we could not test for an organification defect. It is probably a dose effect. Dr. WELLINGTON Moore: We investigated chromosome changes in Chinese hamster thyroid cells after the administration of different doses of 1*1I to animals 7 to 10 days of age. Thirty days and a year after injection the thyroids were removed, trypsinized, and cells grown in tissue culture for 3 days. The cells were arrested in metaphase, fixed, and stained for chomosomal analysis. It was found that thyroid cells were quite sensitive to irradiation. Doses as low as 6 rads produced a tenfold increase in the number of cells containing aberrations at 30 days (3.3% versus 0.31% in the contro! cells). ‘Twenty-five percent of the thyroid cells contained aberrations at a dose of approximately 1,000 rads at 30 days. After 1 year the aberration rate had decreased by three- to fourfold. This was due in part to the increase in gland size associated with the normal growth of the animals. The persistence of the aberrant thyroid cells (approximately one third of the life span) suggests that some of these damaged cells may persist for the remainder of the animal’s life. Dr. Rossins: Thank you for the comment. That is another area I did not get into because of time. It is of interest that Drs. A. A. Al-Saadi and W. H. Bierwaltes at the University of Michigan School of Medicine have been studying chromosomal aberrations in animals and find aberrations resulting from iodine deficiency and other forms of thyroid stimulation, so they may not be limited to radiation effects, although certainly that could be a contributing cause. Dr. Conarp: Attempts were made to do chromosome studies on cultured thyroid tissue from the operative specimens on the Marshallese. Unfortunately, they were not successful. Annals of Internal Medicine NATIONAL INSTITUTES OF HEALTH CLINICAL STAFF SUMMARY The presence of radioactive isotopes of iodine in fallout from atomic explosions is well known. In the accidental exposure of the inhabitants of one of the Marshall Island after a fusion-type explosion in 1954, thyroid radiation from radioiodine, _ particularly in the children, was sizable. This resulted in the late development of thyroid insufficiency or thyroid nodules or both in 80% of those children exposed at less than 10 years of age. At the present time, the thyroid effects constitute the only important overt medical problem in the exposed population. Detection of thyroid damage was hindered byits insidious onset and by the unexpected occurrence of elevated blood levels of iodoprotein in the Marshallese population. The greater preva- } lence of thyroid abnormality in the chil- § dren is probably due to two factors: a larger radiation dose due to small thyroid size and a greater likelihood of the growing organ to manifest changes resulting from chromosomal alterations. The spectrum of radiation exposure of the thyroid glands in any population is very wide, ranging from stratospheric fallout from atomic explosions and clinical testing with radioiodine isotopes to high radiation rates intentionally produced in therapy of thyroid diseases. The Marshall Island experience will help to define the range in which significant thyroid damage may occur. ACKNOWLEDGMENTS Weare deeply indebted to the many members of the medical survey teams over the past 12 years for their participation and for the material presented here. We are also grateful to many other people for invaluable advice and assistance, including Drs. C. L. Dunham and H. D. Bruner of the Atomic Energy Commission: Drs. V. P. Bond, E. P. Cronkite, H. A. Johnson, L. K. Dahl, J. E. Jesseph, and H. L. Atkins of Brookhaven National Laboratory; Drs. S. Warren, B. P. Colcock, and W. A. Meisnner of Boston; Drs. G. H. Klinck, E. D. Henley, C. J. Stahl, C. A. Broaddus, S. Lindsay, and A. Hicking; and to other members of the