-10- amount of cross-contamination between the radioactive elements separated by a combination of the Dowex 50 cationexchange method and the Dowex 1 chloride complexing tecmique. The results of the radioisotopic separations are summarized in Tables 1 end 2. In these samples the predominant fission-product radioisotones were the relatively short- lived Zr?9-Nb?5 (65 a), Cel44-prl44 (285 a), and in one sample, RuLIE_-Ryl 26 (l yr). The fission products accounted for 22 to 37 per cent of the total radioactivity in the three analyses from which cerium determinations were made. In the other two samples fission products contributed at least 5.7 per cent (stations 57,58,59) and 44 per cent (stations 13,14, 15) of the total radioactivity. gr90-¥99 were not detected in the ion-exchange separations nor were they found in fuming nitric acid precipitations made on duplicate aliquots from both the five samples and their filtrates. Of the non-fission radioisotopes, Fe2>, Co37, Co>S ana zn©5 contributed almost all of the activity. Although C960 was present at an average level of only about one per cent, it has the longest half life (5.3 yrs) of the reported nonfission radioisotopes and is therefore important. ; DOE ARCHIVES The variability, mentioned previously, between the retios of Co! to 2n65 also was evident between Co>’ and C000, and between Co>? and Fe22. In Figure 4a, the ratios of Co57/co©° in the five plankton samples are shown; also, these