-~ 16 - colloidal dispersion (<0.2u) and was present in the filtrate fraction of the water samples. Zr95-Nb95 was also found at higher levels in the filtrate fraction than on the filter papers but the ratio was not as high as in the case of wi8s | In an average of the five samples approximately two per cent of the wi85 and 30 per cent of the Zr95-Nb9> remained on the filter papers , The distribution of wt85 and zr95-nb95 on the filter papers and in the filtrates at the time of analysis probably does not represent the condition at the time of collection. Some leaching of radioisotopes would have occurred in planktonic organisms which died during the interval between collection and analysis. A comparison of the distribution of radio- activity in the above samples with that in aliquots of the same samples filtered at the date of collection indicates that this may be the case. The data are as follows: Per cent of activity on filter Water sample stations No. 3 Filtered at date of collection 32 Filtered January 13, 1959 5 No. 31 13 2 No. 37 17 O.1 No. 38 12 0.2 No. 39 25 5