Isotopes of a given element behave similarly in their uptake and metabolism by organisms, except the isotopes of hydrogen and possibly some of the other light elements. Thus one may assume that a radioactive isotope of a given element will follow the stable isotopes of the same element through the food web. Because different isotopes of the same element generally behave identically, the degree at which a given radioisotope is taken up and retained by an organism depends to a large part on the amount of the stable counterpart of the same element present in the environment. The inhibition of uptake of a radioisotope by a stable isotope of the same element is known as isotope dilution, and is important in regard to health hazards from radioelements introduced into the sea. Moreover, elements closely related to each other in their ionic properties will tend to behave similarly and thus simulate isotope dilution. In this way the presence of stable calcium tends to reduce the uptake of radioactive strontium by organisms, and naturally-occurring potassium would be expected to have a similar effect on the uptake of radioactive caesium. Naturally-occurring potassium, which is similar chemically to caesium, is present in sea-water at the relatively high level of about 380 ppm. In comparison with potassium, Cs!*?7 would be present in extremely small amounts by weight even in an area of heavy fallout. Because animals exhibit a limited ability to differentiate between the two elements, the uptake of caesium would be low in the presence of an abundance of potassium. Natural calcium occurs in the sea at a level of about 440 ppm, and is very similar to strontium with regard to uptake by marine organisms. In the case of strontium and calcium, however, the uptake of Sr® is not directly proportional to its occurrence in the water. Sr® is discriminated against with reference to and in the presence of calcium by factors of about 3 in calcareous algae and foraminiferans, 2 in arthropods, 2—7 in molluscs. 2.5 in bryozoans (11), and 3—10 in marinefish (12). In addition to chemical competition, isotope dilution by stable strontium would result in reduced uptake of Sr® by marine organisms. Stable strontium is present in the sea at a level of 6to 1,300 times that of the naturally-occurring forms of the other major radicelements represented in fallout. As a consequence, the isotopic dilution of radiostrontium would be at least 6 to 1,300 times that to which the other radioelements would be subjected because of the presence of their stable counterparts. The apparent discrimination by marine organisms against the uptake of radiostrontium may also be enhanced by a purely physical factor, including scavenging and coprecipitation. Granules of calcite resulting from pulverized islands effectively scavenge radioactive strontium as they sink through the sea into the deeper waters (36). Surro, Takryama and Uyerpa (13) reported that the ashes from the 1 March 1954 detonation which fell on the No. 4 Fukuryu Maru consisted of white granules of calcite approximately 100-400 u in diameter. Identification of the chemical form was madeby electron microscopy and X-ray diffraction techniques. Jt is usually assumed that particles sink in the sea according to Stoke’s Law, in which the assumption is made that the particles are smooth andrigid, are of given diameter and density, and do not interact with each other as they sink through the water. Calculations by Miyoshi indicate that particles of calcite 400 » in diameter would sink from the surface to 260 m in 26 minutes, and particles 100 u in diameter in 7 hours. Calcium from the pulverized islands 118 . yy tteke et Se