RADIATION STANDARDS, INCLUDING FALLOUT 339 experiment. There were 50 separate replications of the experiment, each of exactly balanced irradiated and unirradiated control cultures and each replicated experiment coded so that the scorers were unaware which series had been irradiated. The number of mutations foundin the irradiated series exceeds that in the controls by slightly more than was originally predicted on the basis of the mutation frequency at doses of a thousand roentgens and above. The differenceis statistically significant, so that one may conclude that even doses of 5 roentgens produce mutations at a frequency falling right on the linearly proportional dosage curve. There is no sign of a threshold or of a diminishing effectiveness at low doses. Sterility, or loss of fertility through killing of germ cells or the production of dominantlethal effects that kill the offspring, may also represent genetic effects. In the Drosophila experiments with 5 roentgen doses, it was found the irradiated parents produced significantly fewer offspring than the unirradiated control parents of the same strain, bred in the same number and underthe same conditions. The reduction amounted to slightly more than 1 percent. Even low doses may, therefore, produce a proportional reduction of fertility. In mice the situation is quite different and seems to be more complicated. In male mice, following doses of 100 roentgens or more, temporary sterility results. But after passage of sufficient time, the fertility is recovered. In female mice the reverse is true. After acute radiation, a female mouse may produce one or a few litters, but even a dose of 50 roentgens leads generally to permanentsterility. If the radiation is administered at a low dose rate, it takes only a moderate increase to 80 roentgens to produce the sameeffect. If human females respondedto radiation in this way,thesterilizing effect. of radiation would be the most fearful aspect of exposure to fallout and residual radiation among survivors of a nuclear attack on a population. Studies at the Oak Ridge National Laboratory by the Russells and their coworkers, especially E. F. Oakberg, clarify certain aspects of this situation. Female guinea pigs and hamsters, as well as monkeys, are much more resistant than female mice to the sterilizing effect of radiation. It seems that in the female mice the oocytes in the ovary go into a prolonged arrest in development at a more sensitive stage in their maturation process than is the case in the guinea pig. This difference probably holds true for the other species mentioned and also for the dog and human female, for which evidenceexists that sterility in the female is produced only by much higher doses. Nevertheless, a word of caution seems due. It has been estimated that in the event of a nuclear attack on the United States of 3,000 megatons or more, most survivors will receive an accumulated dose from fallout of upward of 200 roentgens, most. of it in the postshelter period and at relatively low dose rates. Under these conditions the major effect. upon the surviving population might result from the sterilizing effect upon the females. : Weneed much more information about this matter than now exists, in particular to determine whether the dose level that produces complete or partial sterility at low dose rates is significantly lower than the dose level that produces radiation sickness and death. MAMODTR Unt delenit ORCARERCT Ticks LeaBEE reais