SeLeeae -~19- amount of radioactivity was found in the samples taken below the thermocline, Plankton The radioactivity measurements of the plankton samples are given in Table 2. A small amount of beta activity found in the first four Samples can be attributed to residual contamination from previous tests. Because of tne thousand-fold increase in radioactivity of plankton sample P-6 collected on May 6, 1958, as compared to those collected previous to this date, it is assumed that this increase in radioactivity was due to the fallout of the Cactus device detonated on the same day. Samples P-7 to P-12, collected before the Wahoo test, contained radioactivity in lower amounts (Fig. 2). These samples were examined by gamma Spectrometry immediately after collection and the following short-lived isotopes were present in the decreasing order of their contribution to the total activity: Np239 (2.3 jays) Mo99-T.99m (66 hours),* Cei4t_ pr!41 (33.1 days)* Ru 103-Rn103 (39.8 days)* Bal40-:,a140 (12.8 days); Te 132-7132 (77.7 hours)* Zr95-Nb99 (65 days)* Cel44-pr144 (285 days)* and Rul96-Rnh196 (1 year)* in both the large and small plankton. A summary of the results of separations of the radionuclides in sample P-6 by ion-exchange methods is given in Table 3. Gamma spectrum analyses indicated that plankton samples P-7 to P-12 contained these radionuclides in the same relative proportions. *Effective half life. . : ee yor