20-minute background counts were taken during a 24-hour period. From November 24 to the end of December, 127 background counts were made with counter No. 185, and the mean value was 50.25 c/m with a standard deviation of 2.63; for the other counter, No. 184, 135 background counts were made during the same perlod with & mean value of 53.51 c/m and a standard deviation of 3.84. These background values are less than those recorded in the 1949 report because the counting chambers are now shielded with two inches of lead. Decay. At the time of counting, the post test samples were decaying at an appreciable rate. Therefore these samples were corrected for decay and the date to which they were corrected arbitrarily chosen as December 1, one month after the Mike test and also near the mid-point of the counting period for these samples. Although the activity of the samples is corrected to that of December 1, the distribution of activity is that of the date of collection, November 2 to 10. In the period immediately following the shot the activity in the or- ganisms would be expected to vary greatly within short periods, due to changes in geographical distribution of the radioactive materials and to length of time of exposure of the organisms. The curve from which the correction factors were determined was the deeay curve for & sand sample that had been dredged from Rojoa and Aaraanbiru on November 7 at a depth of 30 feet. The principal reason for selecting this curve was that, by inspection, the “composite” of 91 decay curves from various types of organisms and tissues closely resembled the sand - 13 -