Radiochemical separations were made on plankton samples collected in and vest of the Enlwetok Proving Ground in September 1956. Ion exchange resin column and precipitation techniques were used. Fission products, mainly Zr95-ifb^5 and Cel^^-Pr^^^, contributed an average of 29 per cent of the total radioactivity. The remaining 71 per cent of the activity was contributed by the non-fission radioisotopes Zn^5, Co57, 58,60, Fe55 and Mn5^. Radioactive zinc, cobalt, and iron accounted for averages of 2k, 26 and 21 per cent respectively of the total radioactivity. Mn^^was present in trace amounts. Variations in ratio of occurrence for the different non-fission products with change in geographical location was observed. Relatively high levels of Zn"5 were centered in the area near Bikini and Enlwetok Atolls. The area of high levels of radioactive cobalt and iron in comparison to Zn"5 was located approximately 480 miles west and slightly north of Enlwetok Atoll.
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