-23Three of the radioactive non-fission-product elements, zinc, cobalt, and iron, contributed an average of 71 per cent of the total activity in the plankton samples. This orservation 1s in contrast to those on terrestrial samples in which these isotopes were absent or, at most, present in trace amounts only (16). Thus, either a concentrating mechanism must function in the sea for the non-fissionproduct elements, making them available to the organisms, or an exclusion mechanism must operate on land. The ex- tremely low levels at which these isotopes occur in dirt samples near the target area suggest that concentration occurs in the sea. In the case of these radioelements the previously discussed factors that control uptake by marine organisms, with the possible exception of scavenging action by calcium hydroxide or calcite, would tend to cause increased uptake. Thus, these elements in fallout probably occur in the sea in particulate form, are not subject to chemical competition by similar elements or to appreciable isotope dilution by their stable counterparts, nor would co-precipitation occur in the strict sense of the word. However, negatively charged cobalt, menganese, and zinc would tend to precipitate with iron into a finely divided form, exhibiting only a limited tendency to sink below the thermocline and thus remain available to the plankton in the mixing layer. DOE ARCHIVES The isotopes Mn54, Fe55, C057, C058, C09, ana gn©5 are transition elements and have characteristics of variable 2%