tion in the exposure rate and attributed it to weathering during the first 25 days post BRAVO in certain areas of the Marshall Islands (6162). To estimate the reduction in exposure rate due to weathering at Rongelap Island it was assumed the measurement taken at 2.2 days by the USS PHILIP survey team was for unweathered fallout and that the fallout was decaying with the exponent m = -1.4 from day 2.2 out to day 26. This value for m, the decay exponent, was the mean value calculated for the nuclide mixture reported by Yamatera and Tsuzuki for the period 2.2 to 26 days post detonation. This calculated value of the decay exponent was based on the decay of 142 nuclides given in Table 5. The estimate accounted for the contribution to expo- sure rate from 1) the transuranic nuclides U-237 and Np-239, 2) the neutron induced nuclides $-35 and Ca~45, 3) the day 26 fission products which had fractionated according to the mean of the Japanese data (Ya56, Ts56) and 4) the fission product and transuranic product precursors which were present on day 2.2. This decay exponent and the measurement data of the USS PHILLIP crew resulted in an adjustment for weathering losses which increased the exposure rate reported by Held from 40 to 47 mR nv) on day 26 at Rongelap Island, an 18 per cent increase. This was the estimated mean unweathered exposure rate which should have existed on day 26 had the rain storm not occurred. b. Areal Activity at Fallout Cessation Using the estimate of 47 mR h~ 1 as the value for the unweathered average exposure rate on day 26, the unweathered average activity per unit area on Rongelap Island was calculated for the nuclide mixture present on day 26. This estimate was made by multiplying 47 mR no? by 1000 and by the value for nuclide activity per unit area per unit Bikini Ash exposure rate as given in Table 4 colum 4. Although the magnitudes of the uncertainties in the 16