44 cytes. Slight depression of red blood counts, hematocrits, and hemoglobin has beennoted at times. Bone marrow smears taken at 6 months showed no gross abnormalities. Smears taken at 8, 9, and 10 years showed an alteration in the myeloid-erythroid ratio manifested by an increased numberof red cell precursors. Depression of peripheral blood elements in the Ailingnae and Rongerik groups was not so pronouncedas in the Rongelap group. However, a slight lag in complete recovery in the Ailingnae peripheral blood count has also been noted. The persistent depression of peripheral blood elements in the exposed people makes it appearlikely that there is slight residual bone marrow damage. A general anemic tendency has been evident in both exposed and unexposed Marshallese. PriceJones curves, on the average, showed slight microcytic tendency. Serum iron levels have been generally normal, and the cause of this anemic tendencyhas been undetermined. Reticulocyte counts have been about the same in the exposedas in the unexposed. Clinicalfindings, except for radiation-induced lesions of the skin, patchy epilation, and early gastrointestinal symptoms, revealed no clear-cut disease processes or symptoms which could berelated directly to radiation effects during thefirst few years post exposure. No prophylactic or specific therapy for radiation effects was ever considered necessary or given. Epidemics of chicken pox and measles that occurred during the first year showedno greater incidence orseverity in the exposed than in the unexposed Marshallese people. During thefirst months post exposure about % of the exposed people exhibited loss of weight of several pounds. This may possibly have been related to their radiation exposure, although it was difficult to rule out possible effects due to change of environment. At 3 years post exposure the immune response to primary and secondary tetanus antitoxin was tested and found not to be significantly different in the exposed compared with the unexposed populations. Beta Irradiation of the Skin It was impossible to get an accurate estimate of the radiation dose to the skin. Beta burns of the skin and epilation appeared about 2 weeks after exposure, largely on parts of the body not covered by clothing. About 90% of the people exposed on 500630! Rongelap had these burns, and a smaller number developed spotty epilation of the scalp. Most of the lesions were superficial; they exhibited pigmentation and dry, scaly desquamation, and were associated with itching and burning sensations. Rapid healing and repigmentation followed. Some lesions were deeper, showed wet desquamation, and were morepainful. A few burns becamesecondarily infected and had to be treated with antibiotics. Repigmentation of the lesions gradually took place in most instances, and the skin appeared normal within a few weeks. However, in about 15% of the people, deeperlesions, particularly on the dorsum of the feet, continued to show lack of re- pigmentation with varying degrees of scarring and atrophy of the skin. Numeroushistopathological studies have been made,!-45 and the changes found have been con- sistent with radiation damage. Spotty epilation on the heads was shortlived, regrowth of hair occurring about 3 monthsafter exposure and complete regrowth of normal hair by 6 months. No further evidence of epilation has been seen. An interesting observation noted during the first few months after exposure was the development of bluish-brown pigmentation of the semilunarareasof thefingernails and toenails in about 90% of the people exposed on Rongelap. By 6 monthsthis pigmentation had disappeared, having grown out with the nail. The cause of this phenomenonis not known. Internal Irradiation Radiochemical analyses of numerous urine samples of the exposed population showedinternal absorption of radioactive materials, probably brought aboutlargely through eating and drinking contaminated food and water and to a lesser extent through inhalation. Duringthefirst few days when the bodylevels were at their highest, the maximum permissible concentrations were approachedorslightly exceeded only in thecase of strontium-89 and the isotopes of iodine. At that time the concentrations were believed to be too low to result in any seriouseffects. Body levelsfell rapidly, so that by 2 and 3 years post exposure they were far below the accepted maximum permissible level; even by 6 months activity in the urine was barely detectable. No acute effects were observed that could be related to internal exposures. As seen later, however, the significanceof