about 13 million mutation-bearing conceptions of the lethal variety. A small but undetermined proportion of these will be expressed as lethal dominants and will be eliminated in the first generation, many as early miscarriages. Approximately 4% of the recessive lethal genes will also come to expression as genetic deaths in the firat generation and these will amount to approximately 20,000 conceptions per year out of the expected 2,700,000 births per year. If we add to this the ex- pected genetic deaths from the detrimental mutations, twice the number of lethals with an assumed maximm of 4% for the detrimental factor, there can be a total of 60,000 genetic deaths per year for the first generation out of the expected 2,700,000 births per year. This rate will decrease with time until the total number of genetic deaths equals approximately one-half to one-third of the 13 million lethal plus 26 mil1ion(detrimental)mutations. It is worth pointing out that an informal exercise at RAND which assumed 150 15 Mr kreapons dropped over the United States, east of the Mississippi, resulted in the delivery of . approximately 86 r/person to the total population of that area, i.e. , east of the Mississippi, on the further assumption that shelter giving 90% protection was available and used by everyone. This condition would reduce the totals given in the example above by a factor of 2, because of the fact that the immature germ cell mutation rate would apply for the bulk of the radiation exposure. The amount of additional genetic damage caused by radiation after people leave their shelters will depend on decontamination, weathering, gross terrain shielding, and other factors. It appears, therefore, that a large amount of radiation will not present an inordinate genetic effect as long as the radiation is applied to one generation only and is not repeated. Repeating the dose discussed above, generation after generation, would before many generations reach a new mutation rate which could be incompatible with species survival. 92