INTERNAL IRRADIATION Radiochemical analyses of numerous urine sam- ples of the exposed population showed internal absorption of radioactive materials, probably brought about largely through eating and drinking contaminated food and water and to a lesser extent through inhalation. During the first few days when the body levels were at their highest, the maximum permissible concentrations were approached orslightly exceeded only in the case of strontium-89 and the isotopes of iodine. The concentrations were believed to be too low to result in any scrious effects. Body levels fell rapidly, so that by 2 and 3 years post exposure, they were far below the accepted maximum permissible level; by 6 months activity in the urine was barely detectable. In 1958 analyses of bone samples on one of the men who died showed 3.7 strontium-90 units/g calcium. Beginning in 1957, gamma spectroscopy by use of a low-level counting chamber was added to the techniques of radiochemical analysis. The return of the Rongcelapese to their home island (which after careful survey was considered safe for habitation, despite a persisting low level of radioactive contamination) was reflected in a rise in their body burdens and increased urinary excretion of certain radionuclides. During the years since the original contaminating event, additional weapons tests held in the arca have contributed to the fission products in the environment. Since the dict includes a variety of imported foods, the people are not living in a “closed” environment, and therefore may not be rapidly approaching equilibrium with the environmental fission products, as might be expected under other circumstances. Body burdens of gamma-cmitting fission products (such us Cs"? and Zn‘*) were measured in a whole-body counter and checked by radiochemical analysis of urine specimens, The levels of internal contaminauion per unit weight appeared to be about the same for juveniles as for adults, male and female. Wide vaaiations in levels of contaminaGon in any group were found, apparently due to differences in dict and metabolism. Body burdens of Sr” were estimated from urinary excretion as determined by radiochemical analyses. Both the external dose measurements on Rongelap Island and the levels of radioactive isotopes in the food on the island indicated that some increase in Cs'*7, Zn", and Sr*” body burdens was to be expected when the people returned there in 1957. The Cs" body burden in 1958 was about 0.68 »C, about 60 times as greut as in 1957, and the urinary Cs'?? level rose by a factor of 140; the mean body burden for 1959 was 0.57 wC. The mean body burden of Zn™ estimated from wholebody counting data was, in 1958, after the return to Rongelap, 0.36 uC, 8 times as high as in 1957, and 0.44 pC in 1959, In 1961 the mean Cs'*" body burden in adult males was 14.7 muC/kg, which is not significantly different from the mean value of a similar group obtained in 1959; it was 300 times that of the medical team, who were measured at the same time for comparison. The Zn** level in adult males (1.51 myC/kg) dropped to 17% of the mean value measured in 1959, With a larger detector and 2 longer counting time than previously employed, it was possible to identify and quantify Co™for the first time in these people; the mean level of Co™ was about 11% of the Zn® level. A smal! amount of residual activity was still present after the subtraction of K* and the above radionuclides from the total spectrum. The mean level of urinary excretion of Sr was 7.2 puC/! or 14% higher than measured in the 1959 medical survey. Little of the body burden of the exposed group is apparently duc to their initial exposure, since at present thereis little difference between the levels of the exposed and unexposed populations living on Rongcelap Island. The body burdens are of small significance in terms of radiation hazard. OTHER STUDIES Studies of genetically inherited charucteristics. Blood grouping studies in the Marshallese showed a relatively high B gene frequency, a high N gene frequency, an extremely high R' gene frequency, and total absence of Kell and Dicgo factors. These characteristics differ from those of Polynesians and suggest relationship with Southeast Asians and Indonesians. Haptoglobin studies showed the fre- quency of the Hp' gene to be higher than in European populations thus far tested and consistent with populations living near the equator. The distribu- tion of haptoglobin types showed the population to be relatively homogeneous. Transferrins in all sera were tvpe CC, the common European type. f-Amino-iso-Autyric acid urinary levels showed the Marshallese to be the highest excreters of this acid of any population thus far reported. Levels in the exposed group were about the same as in the unexposed group, and no correlation was found with