from which samples were combined for analysis. Five plankton samples were analysed in detail for their radioisotopic content eleven months after the date of collection. These included Sample No. Stations I 7, 8, 9 II 13,14,15 III Iv 43,44 45,46 50,51,52,53 57,58,59 V The samples were filtered through No. 1 Whatman filter papers and the plankton and filtrate fractions dried separately. Gamma spectra were made from both fractions. The plankton samples were then wet ashed with concentrated nitric acid and hydrogen peroxide and converted to the chloride form by evaporating the sample to partial dryness repeatedly with hydrochloric acid. The moist salts were dissolved in 0.2 N HCl, filtered, and the filtrate passed through a cation-exchange column (5 mm x 100 mm) of Dowex 50 (13, 24, 12, 16). DOE ARCHIVES Two hundred and fifty mgm or less of the sample in 0.2 N HCl solution were passed through the column at 4 rate of 0.2 0.5 ml per minute, 0.2 N HCl. then the resin was washed with 50 ml of The eluate from the sample and the subsequent hydrochloric acid wash contained those ions not adsorbed to the resin, i.e. the anions. These included Zr95-Nb95 and/or