noting that many of these early terminations of fertilized egg cells may represent e natural mechanism for removal of many abnormal conceptions. Although purely speculative, this concept does support the possibility that a large proportion of the artificially mutated genes would be eliminated by nature before becoming a significant social burden. B. Evaluation of the Human Hazard Due to Radioactive Fall-out: Internal. The external hazard due to fall-out radiation is essentially confined to the local fall-out area. The internal hazard, i.e., the hazard created by internal deposition of various isotopes in humans, exists both in the local fall-out area and world-wide. Quantitative aspects of fall-out and fractionation of critical elements suggest that the hazards to be expected locally are very great compared to the world-wide problem. If it is assumed that local fall-out areas will be large because of large numbers of bombs being dropped, then the local area hazard is likely to be the critical consideration for internal as well as external radiation injury. . Criteria for Assessment of Biological Significance. All of the radicactive components of fall-out material are hazardous to some extent. It is not necessary, however, to evaluate in detail the radia- tion injury due to each of these components. In a general assessment of internal hazard it is sufficient to find the component or components which are responsible for the major part of the hazard. From the point of view of safety, the most critical element becomes the limiting factor and any standards set for the critical element automatically remove the others from need for consideration. In estimating effects above the safety level, it becomes necessary to add all elements which are of appreciable significance in their contribution to the total effect. As will be shown below, Strontium-90 appears to be not only the critical element for determining safety standards, but it is also responsible for practically all of the long-term effect, and therefore the parameters relating to this 95