ratios of adult average body burden for 6520, 905,, and 13765 were used in the calculation. Ratios were estimated for the period after the Ronge lap adult body burdens reached a maximum value. The Rongelap-to-Utirik ratio, 2.6 + 0.39, has been relatively constant since 1958. The initial increase in 1958 in the 13765 average body burden for Rongelap adults (see Fig. 2) was due to dietary intake of 13764 and a small intake of 137¢5 from the air and water due to above ground nuclear tests in the Marshall Islands during 1958. The subsequent drop in the 1959 137 Cs body burden may have been due to increased use of imported food and the conclusion of the testing. The reason for an increasing 1376, body burden at Rongelap during the 1960's was uncertain. Residual contamination from the Hardtack weapons testing program and subsequent incorporation of 1376, into diet items was one hypothesis. The Hardtack Phase I series of tests was conducted during 1958, just prior to an increase in the exposure rate at Rongelap Atoll (Un59). Small amounts of fallout from the CACTUS, YELLOW WOOD, and HICKORY experiments in this series: reached Rongelap. However, several observations support the conclusion that 13764 from this series was insignificant relative to 13766 from the Castle seties. First, the peak 1376, body burden of a similar population at Utirik occurred three years after the initiating event (Castle BRAVO in 1954) while the 1965 peak 1376. body burden at Rongelap followed the Hardtack series by seven years. Secondly, the peak exposure rate on Rongelap which occurred during the Hardtack series in 1958 was about 10,000 times less than the peak exposure rate following BRAVO. These facts suggest that debris from the Hardtack series was not a major factor influencing the Rongelap 1376, body-burden pattern during the mid 1960's. In addition to Hardtack series fallout, the adult average body- burden pattern would have also been influenced by 1) world-wide fallout 13