ss contamination from previous tests is always present in the environment. However, comparisons of levels of radioelements in samples collected at different times do show the overall trend of uptake of radioactive materials by these organisms, and the observed ratios of the different radioisotopes in the organs of inidividual specimens indicate the sites of deposition in organs andtissues of the fish, and in some instances give evidence concerning turnover rates. In Table III the results of radiochemical analyses on organs and tissues of fishes from the Eniwetok Test Site are shown. In a yellowfin tuna sample from Eniwetok Lagoon, taken during the testing period in June 1958, Fe*> was the predominant isotope, with red muscle, kidney and spleen containing more than 70% of the total radioactivity as this isotope. The highest level of Co5’. 58. 6 was in the kidney, suggesting a high turnover rate for radiocobalt in this fish. The low value of Zn®> in the kidney suggests a low excretion rate for this element. The high level of radioactive iron in the kidney is probably not caused by a high excretion rate, but rather indicates a concentration of iron, which would be expected, since the kidney in fishes is a blood-forming organ. Similarly a high level of radioactive iron in the spleen is probably the result of active concentration of iron, since the spleen, too, has haematopoietic functions. In the red muscle Fe®® contributed 70% of the total radioactivity, but in the white muscle only 33%. Red muscle is known to contain greater concentrations of iron than white muscle, and therefore the deposition of radioiron in this organ would be expected. Mn*4 apparently is not concentrated in the red muscle, white muscle, kidney or spleen, and was found in the liver at a level of only 4% of the total radioactivity. In fish-liver samples collected from Eniwetok Lagoon in September 1956, approximately six weeks after the test series, Zn®* contributed 47% and Fe®> 35% of the total radioactivity. Radioactive cobalt was present at a level of about 14% of the total radioactivity, about the same percentage as that observed in the liver of the yellowfin tuna collected during the tests. Thus in fish-liver samples collected six weeks after the test series in 1956, the Zn®> was slightly higher and the Feslightly lower than that found in yellowfin tuna liver collected during the 1958 test series. In the livers of reef fish collected at Ailinginae Atoll during July 1957, approximately three years after contamination, the levels of radioisotopes were similar to those in the two liver samples from Eniwetok collected during the test period and six weeks after. During the test period in the summer of 1958, tuna samples taken in the open sea near the test site had different ratios of radioisotopes from those taken in the lagoons. In these samples Zn** contributed 81—91 % of the total radioactivity in five different organs and the levels of Fe®5-59 ranged from 5 to 18 %. Thus, in the fish samples collected in the open sea, the level of Zn® in relation to the total activity is approximately twice that found in the lagoon fishes, and the levels of Fe5* and Co‘. 58. 6 are correspondingly lower. In addition, whereas small amounts of Zr°5—-Nb®* were present in some of the lagoon samples, this radioisotope was not detected in the tuna samples from the open sea. The plankton samples collected during the Collett survey (Table I) were taken at approximately the same time as the tuna samples from the open sea near the Marshall Islands (Table IIT). A comparison of the levels of the 126