solved radioactive material, may accumulate on the surface of plankton organisms, and that in addition there are larger par- ticles (the cheesy material) suspended in the water. Algae. In order to evaluate the "speck" contamination of algae, autoradiographs of washed and unwashed specimens were made. Washing was done by scrubbing with a brush and detergent and was followed by rinsing with running tap water. Autoradiographs of an alga, Udotea, before and after washing, are shown in Figure 2la and b. Some of the radioactive spots were removed by washing, but most of them were not, showing that a major portion of the radioactivity is actually present within the alga. The even distribution of radioactivity in the filaments of Lyngbia and in the ramuli of Bryopsis, shown in c and d of this figure, indicate that “speck” contamination is of minor importance in these specimens. In one alga of the pre- shot collection (Fig. 2le) adhering soil particles were res- ponsible for numerous hot spots in the autoradiograph. The autoradiographic method has indicated the presence of both sur- face and absorbed contamination in the algae collected before and after Mike shot. The relative amount of speck contamina- tion was high in some cases and low in others; however, & quan- titative estimation cannot be made. Invertebrates. Among the invertebrates, an outstanding example of spotty distribution of activity was the occurrence on a piece of coral of the genus Acropora, taken at Bogallua November 8, 1952, of 3 highly radioactive nodules firmly at- - 92 -