changes from the gamma radiation of the particles not in direct contact with the individual. There were indications of small but measurable quantities of internally deposited radioactive isotopes. Animal experimentation has shown that external irradiation to the gonads can cause gene mutations. Most of these mutations are deleteri- ous and may cause eventual genetic death. In order to assess the sig- nificance of this phenomenon the basic genetic data have been extrapolated to man as realistically as possible in the light of current knowledge. The basic biological extrapolation has been interpreted in terms of the mown or predicted physical radiological exposure data. All damaging radiations from fall-out are attenuated in scme degree by matter interposed between the source and the receiver. Smell amounts of mterial, such as a layer of heavy clothing, will absorb almost completely the alpha and beta radiations from fall-out. The gammradia- tion from fall-out cannot be completely stopped by shields, but even minimal shielding can be quite effective in reducing gamma radiation received to tolerable levels. Thus it can be seen that the phenomonology of nuclear weapons is such that the physicaleffects determine the biological parameters to be evaluated. The hazard created in the close-in fall-out area by a large surface detonation is greater by several orders of magnitude than was ever observed until the high levels of fall-out radiation were dis- covered following the Bravo shot ofthe CASTLE series. The biomedical data presented indicate that absolute safety and freedom from injury are unattainable since some radiation effects such as genetic damege and possibly carcinogenesis do not have a threshold of injury as far as can be determined. It must be borne in mind that the population at risk is larger by far than was ever considered in programming for adequate public health practices or disaster handling for the cowutry at large. A good educational program regarding the hazards involved, leading to utilization of the beneficial effects of all available shielding, early evacuation and adequate decontamination procedures will ayold many unnecessary casualties should such a catastrophy as a nuclear war ever occur. : k