-27- Summary 1. During July 1957, chemical separations by ton-exchange resin columns were made on five grouped plankton samples taken in and west of the Eniwetok Proving Ground in September 1956. Also, radiochemical separations for Sr99-y90 were made. The radioactive isotopes in the separated fractions were identi- fied by decay rates, maximum beta energies and gamma energies. 2. The distribution pattern of Co°7 and Zn®5 simples not subjected to in plankton ion-exchange separation was deter- mined by analysis of gamma spectrum curves. 3. The predominant fission product radioisotopes found were Zr95-Nb22, Cett4-prl44 ana in one sample, Rul06-Rp106, Fission products accounted for 22 to 44 per cent of the total radioactivity at the collection date. in the Sr99-y90 was not found lon-exchange separations nor in fuming nitric acid pre- cipitations made on duplicate aliquots. 4. [DOE ARCHIVES The non-fission radioisotopes Fe55, Co57, Co58, ana zn®5 contributed 55 to 77 per cent of the total radioactivity, and Co? ana Mn>4 accounted for a total of 0.8 to 1.6 per cent. 5. Cod? and Co©° were present at Bikini Atoll in a 1:1 ratio and at Eniwetok ina 5.3:1 ratio. These ratios were re- flected in plankton samples collected in the vicinity of the two atolls. The ratio of Co57/¢060 in an area 300 miles wide north to south and 200 to 500 miles west of Eniwetok was 30