SUMMARY A study was undertaken to reexamine thyroid absorbed dose estimates for people accidentally exposed to fallout at Rongelap, Sifo, and Utirik Islands from the Pacific weapon test known as Operation Castle BRAVO. The study included: 1) reevaluation of radiochemical analysis, to relate results from pooled urine to intake, retention, and excretion functions; 2) analysis of neutron-irradiation studies of archival soil samples, to estimate areal activities of the iodine isotopes; 3) analysis of source term, weather data, and me- teorological functions used in predicting atmospheric diffusion and fallout deposition, to estimate airborne concentrations of the iodine isotopes; and 4) reevaluation of radioactive fallout, which contaminated a Japanese fishing vessel in the vicinity of Rongelap Island on March 1, 1954, to determine fallout components. The conclusions of the acute exposure study were that the population mean thyroid absorbed doses were 21 gray (2,100 rad) at Rongelap, 6.7 gray (670 rad) at Sifo, and 2.8 gray (280 rad) at Utirik. The overall thyroid cancer tisk we estimated was in agreement with results published on the Japanese exposed at Nagasaki and Hiroshima. We now postulate that the major route for intake of fallout was by direct ingestion of food prepared and consumed outdoors. We believed urine bioassay results for 1311 excreted from people exposed at Rongelap to be accurate. We extrapolated our thyroid dose estimates from. measured and derived quantities which were related to urine bioassay results. The facts which related were 1) fallout arrival and observation times, 2) size and nuclide composition of BRAVO fallout (also known as Bikini ash), 3) Rongelap, Utirik and Sifo Island exposure-rate measurements and 4) diet and Living pattern observations. The 1311 intake, which we estimated from 131] measured in urine, was used by us as a normalization point to link related facts. This allowed us to estimate the intake of other iodine isotopes and the intake of radiotelluriums. Studies used by us to deduce the amount of 129y deposited at Rongelap and Utirik as a result of the BRAVO detonation revealed a much greater level of 1297 in soil than could be derived by using other methods. Additionally, the uncertainty associated with soil measurements was very great. We concluded that other weapons tests may have influenced the soil concentrations of 1297 and that isobars rather than isotopes of 1297 may have exhibited similar behavior. Thus, using I to derive the intake of other radioiodines was not possible. The meteorological approach used to assess thyroid dose did not result in agreement with relatable quantities. The value for predicted increase in exposure rate based on meteorology, or the meteorological-based estimate of wholebody dose, duration of fallout, or airborne activity concentration were not consistent with values obtained by measurement or estimates derived by different methods. The composition, specific activity and particle size of BRAVO fallout (Bikini ash) were in agreement with other observed facts. Based on BRAVO fallout composition and specific activity studies, surface activity results which we derived for varous locations downwind of the detonation site were in agreement with directly measured surface activities made at these same locations shortly - xiii -