=10= 
amount of cross-contamination between the radioactive ele- 
ments separated by a combination of the Dowex 50 cation- 
exchange method and the Dowex 1 chloride complexing tecmmidque. 
The results of the raditoisotopic separations are summa- 
rized in Tables 1 énd 2. In these samples the predominant 
fission-product radioisotopes were the relatively short- 
lived Zr?5-Nb?5 (65 a), Cel44-prl44 (285 a), and in one 
sample, Rul0€E-Rpl6 (3 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-y990 were not detected in the ion-exchange sepa- 
rations nor were they found in fuming nitric acid precipi- 
tations made on duplicate aliquots from both the five samples 
and their filtrates). 
Of the non-fission radioisotopes, Fe>5, CoD7, CoD8 ana 
zgn©5 contributed almost all of the activity. Although C060 
was present at an average level of only about one per cent, 
it has the longest half life (5.3 yrs) of the reported non- 
fission radioisotopes and is therefore important. 
The variability, mentioned previously, between the 
ratios of Co! to 2n©5 also was evident between Co?! and C060, 
and between Co?! and Fe>. In Figure 4a, the ratios of 
CodT /C060 in the five plankton samples are shown; also, these 
