rediation rates in a fall-out field at times from five minutes to about one year after burst time. The body is known to recover from at least a major portion of sublethal damage due to ionizing radiation. The rate of repair is wn- known, but is believed to be of the order of 10% per day of the reparable damage remaining. Lethal dose estimates are well within the limits of the accuracy of the physical data upon which they are based. In a fall-out area where the gamma radiation dose is sub-lethal, fission product beta and soft gamma emitters in the fall-out are a serious hazard only when in contact with the skin. A high degree of pro- tection against this type of radiation is afforded by clothing which prevents fall-out material from contacting the skin. There is a need for a high yield land-surface test detonation to fill in uncertainties in the present fund of knowledge on fall-out. (v The primary long-term and world-wide hazards arise from the distribution by winds in the upper atmosphere of certain cancer-forming radioisotopes produced in the fission process, and their subsequent biological uptake by humans. Prominent among these are strontium-89, strontium-90 and iodine-131. Approximately one gram each of strontium-89, strontium-90 and dodine-131 is formed per KT of fission yield. Because of fractiona- tion, these elements are produced in such form and quantity as to favor their world-wide distribution as opposed to local deposition. However, for surface bursts the local contamination still remains the dominant factor. The potential genetic effect of fall-out radiation on man is likely to be secondary in importance to short-term effects as well as to the hazard due to long-term cancer-producing effects) The chief uncertainties in this regard lie in the extrapolation of animal data to man. The manner of expression of the bulk of such radiation- induced mutations is, likewise, not certain. If manifested as miscar- riages, this would greatly reduce the possible burden to society involved. 130