-8to those in other tissues. Differences are greatest in vis- cera in which the amount of radioactivity is much greater than in liver tissue the first 100 days; after this period the two tissues decline at about the same rate. Bone and skin fluctu- ate about a common intermediate range between muscle and liver, the muscle always having the least radioactivity on a per-gramwet-weight basis. During the first 100 days all tissues show a decrease in radioactivity of more than 90 per cent from early post-shot levels. By the 250th day the tissues had reached preshot lev- els, which, at Eniwetok, were higher than normal because of other detonations set off in previous tests. The more or less consistent relationship of one tissue to another during the decline of radioactivity may indicate comparatively slight differences in selective uptake in the five tissues. An approximation of the relationship was obtained by dividing the total amount of radioactivity in all the samples of the same tissue by the number of samples to give the average amount present on a per-gram-wet-weight basis for the period of April 1954 to November 1955. The following results were produced: Skin Average uc/ke Percent of total of 5 tissues 11 8.3 Muscle Bone Liver Viscera 1.0 9.8 29 77 0.8 7.7 22.9 60.3 The relationship varies depending on the time after shot and