the nature of the biological consequences involved. Operation CASTLE Experience. Operation CASTLE has provided considerable information on the internal hazard due to fission product fall-out; however, the analyses reported thus far are subject to certain criticisms. If determinations of fission decay products pre- sent in the urine at a certain time post shot are extrapolated back to shot day in terms of body burden to bone and soft tissue, a much larger estimated than actual result will obtain unless the rate of uptake and initial rate of excretion are considered. Further, if a cumulative acquisition of internally deposited fall-out debris from a series of shots is extrapolated back to the date of the initial shot, a similarly larger calculated than actual dose will result. In connection with the first criticism, the total body burden at one day after the shot for strontium-89 and iodine-131 is given as 1.6 microcuries and 6,400 microcuries, respectively. Even assuming strontium-89 to have been received as an acute single dose during actual fall-out, the more meaningful body burden would be the 82nd day value of 0.19 microcuries, representing as it does the actual bone burden. Accepting for the sake of illustration the 1.6 microcurie value for the first day, a comparison of decay rates of strontium-89 versus strontium-90 indicates strontium-89 to have more than 100 times the activity of an equal amount of strontium-90 at day one. On this basis the total strontium-90 burden at day one would be 1.6 x 10°? pe and on day 82, a value of 1.9 x 107? ue uncorrected for strontium-89 decay. This value of 1.9 x 10°? microcuries represents roughly 1.9 SUNSHINE units, a value within the standard deviation of the world-wide value for SUNSHINE units of strontium in bone. In the case of iodine-131 3 the body burden at day one is given as 6.4 x 103 microcuries, and if comparison of the decay rates of iodine-13] versus its shorter-lived isotope iodine-133 is made, the day one body burden of iodine-133 would be on the order of 6.4 x 104 microcuries. These values of both Lodine-131 and iodine-1335 are well within normal: therapeutic dose ranges for 125