-32in the colloidal form. These radioisotopes, with the exception of ce! 48. pri43) were also the predominant ones in the plankton collected after Wahoo, a weapons testing device identical to Umbrella” These data suggest that in the first few days after detonation of a nuclear device these radioisotopes are mostly insoluble and that in this form they are removed from the water by the plankton. In considering the distribution of radioactivity in the sea following a nuclear detonation, the role of the plankton should be considered. To estimate the relative amounts of radioactivity in water and plankton samples, the amount of water strained by the plankton net was calcu- lated in the following manner. The total volume of water filtered was calculated to be the product of the distance the net was towed, the filtering efficiency of the net (50 per cent), and its cross-sectional area. When the radioactivity of the total volume of water that passed through the net was calculated and compared with the radioactivity of the plankton in the same volume of water, the value for the water times greater (Table 9). was found to be several thousand This comparison indicates that the removal of radioactivity from the water by the plankton was apparently a minor factor, at least in the first 48 hours after detonation. The amount of Sr”~ 90 in the samples varied from 0 to 0.04 per cent of the gross beta activity. These values agree well with those obtained 3S < wy oO Owe xv cs