fall-out, and the yield and type of bomb together with the proximity of the detonation to the earth determine the total amount and kind of debris available for fall-out. After local fall-out is completed, the process continues in more remote areas to a lesser and lesser degree over a period of months or even years as the cloud is taken by the wind currents around the world. Particles from close-in fall-out and world-wide diffusion have been collected by several different sampling methods at altitudes ranging up to 90,000 feet. Radiochemical analysis of these particles shows the presence of many artificially radioactive isotopes. The percentage composition of these, allowing for radioactive decay, generally parallels the yield of the element from the fission process, although certain elements have been found to vary from the expected quantity for physically explainable reasons. There are certain radioactive elements which are known to be bio- logically hazardous when deposited in humans and enimals. Examples of these are strontium-89, strontium-90, and iodine-151, all of which are formed in considerable quantities as a consequence of fission. These potentially hazardous fission products may be taken into the body by either inhalation or ingestion, and tend to collect in certain parts of the body such as the bone or thyroid, where their damage is done over extended periods of time. Very little definitive information exists on human response to ionizing radiations, and one must rely largely on animal date, supple- mented by data from a limited number of accidental human exposures, in evaluating the fall-out hazard. . In the CASTLE test series in the spring of 1954, 239 Marshallese natives and 28 American servicemen were accidentally exposed to sublethal amounts of fall-out radiation. Clinical studies of these human subjects proved that quantities of radiation biologically damaging to humans may be received from this type of radioactivity at great dis- tances from the point of origin. The biological damage, although kept sublethal because of prompt evacuation, included beta burns when radioactive particles were in direct contact with the skin, and hematological 3