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The variation in the ratios Co27/zn65 and Co?! /Fe55 in 
plankton samples with change in collection site may also be 
due to differences in levels of the three isotopes in the 
fallout material. However, the possibilities should not be 
overlooked that it may be due to preferential scavenging of 
zine by calcite and calcium hydroxide in the area of intense 
fallout or to biological effects. Data that would resolve 
these points are not évailable and an effort should be made 
to obtain the necessary information in any future operation. 
Evidence that the differences in C057 /zn65 ratios are 
real and not due to techniques of separation and analysis 
is found in the gamma spectrum curves on whole plankton 
samples (Fig. 2). Inspection of the curves from the three 
samples shows clearly the differences between the levels of 
zn©5 and both of the cobalt isotopes, Co57 and C0®9, in the 
whole sample before separations and analyses were performed. 
