Sai 
ratios for other biological samples from within Eniwetok and 
Bikini Atolls, reported in an earlier paper, are given (16). 
Co5T and Co60 were present at Bikini Atoll in a 1:1 ratio; at 
Eniwetok in a 5.331 ratio. A similar difference in levels of 
the two cobalt isotopes also occurred in the plankton taken 
in the open sea in the vicinity of the Pacific Proving Ground: 
Thus, at stations 57, 58, 59, approximately 200 miles northwest 
of Bikini Atoll, the ratio Co57/co®° is 4.6:1.0 and the value 
150 miles northwest of Eniwetok Atoll is 7.6:1.0. The ratio 
of CodT /¢ 069 in an area 300 miles wide north to south and 200 
to 500 miles west of Eniwetok was constant at a ratio of ap- 
proximaitedsy dale: 
The ratios of CoD!/zn©5 for the plankton stations are 
shown in Figure 4b. The values for the five plankton samples 
on which ton-exchange separations were made are shown cross- 
hatched; the remeining CodT /zn®5 ratios for plankton were 
calculated from the gamma spectra on total samples. The 
ratios for Eniwetok and Bikini samples were taken from ion- 
exchange separation data in an earlier report (16). 
The ratio of Co57/zn©5 at both atolls was 0.06:1.0, indi- 
cating a relatively high level of zn©5 with respect to Co’. 
The same condition prevailed at stations 3, 4, 5, and 6, 200 
miles south and west of Eniwetok Atoll. However, the ratio 
north of Eniwetok and Bikini along the northern boundary of 
the restricted zone was slightly higher (0.1:1.0) than that of 
the atolls. The center of high Co?’ /zn©5 ratio (5.5:1.0) was 
