RADIATION STANDARDS, INCLUDING FALLOUT 345 however, that there is no ground, on the basis of Russell’s data, for concluding either that the relation of mutation frequency to dose is not one of linear propor- tionality or for concluding that there is any threshold at very low doses. In both of these important matters, the data show precisely the contrary. For the low His (penalgee dose rates, the frequency of mutations still increases linearly with the dose. There is no indication of a threshold. A total dose of only 86 rcentgens at a low dose rate produced exactly the expected frequency of mutations. Efforts to confirm this important finding of a difference in the effects of high and low dose rates applied to the immature germ cells of the mouse have been made. Work with Drosophila, carried out by Muller, Oster, and Zimmering, leaves the issue uncertain because of difficulties in dosimetry, although the first reports indicated a confirmation of the effect. It is reported that a similar experiment conducted at Harwell has confirmed the dose rate effect for Drosophila. In the mouse it is impossible without vast expense and labor to test the mutation rates for doses much lower than 86 roentgens. For this kind of experiment we must resort to using a species that can be raised more cheaply and in far greater numbers, such, for example, as the fruit fly. In my own laboratory we have recently completely a 3-year study of the mutation frequency produced by a dose of only 5 roentgens to the mature male and female germ cells. This represents, I believe, the lowest dose studied for mutagenic effect in any animal up to this time. During the early days of the Manhattan project a team of expert geneticists spent several years in pushing the dosage curve for mutation down, first to 50 roentgens and finally to 25 roentgens. To those levels the dosage curve was shown to be strictly linear. The present study carries the dose relationship down to a level comparable to that of the normal human 30-year dose from background radiation. In order to carry out this study, dominant mutations of a particular minute bristle type occurring at some 60 genetic loci in the chromosomes were resorted to, instead of the usual recessive lethal mutation technique, which requires breeding a culture for every tested germ cell. In our recent study, 1,360,948 individual flies descended from parents each of which had received a 5 roentgen dose of X-rays were scored. There were 50 separate replications of the experiment, each of exactly balanced irradiated and unirradiated control cultures and each coded so that the scorers were unaware which series had been irradiated. The number of mutations found in 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 1,000 roentgens and above. The difference is statistically significant, so that one is entitled to 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 diminishing effec: tiveness. Sterility, or loss of fertility through killing of germ cells or the production of dominant lethal effects that kill the offspring, usually at a very early stage of development, may also represent genetic effects, although they are often not passed down beyond the immediate progeny of the treated individuals. In the Drosophila experiments with 5-roentgen doses, just described, it was found that the immediate parents produced significantly fewer offspring than the unirradiated control parents of the same genetic strain, bred in the same numbers and under the same conditions. The reduction amounted to slightly more than 1 percent. Even low doses may therefore produce a proportional reduction of fertility. In mice, however, the situation is quite different, at least in the female sex. In male mice, following doses of 100 roentgen or more, temporary sterility results; but after passage of sufficient time for the more sensitive spermatozoa, spermatids, and cells in the maturation divisions to be replaced by cells that were in the more resistant spermatogonial (immature) stages when they were irradiated, 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 roentgen leads generally to permanent sterility. If the radiation is administered at a low dose rate, it takes only a moderate increase (to 80 roentgen) to produce the same effect. If human females responded to radiation in this way,the sterilizing effect of radiation would be the most fearful aspect of exposure to fallout and residual radiation among the survivors of a nuclear attack on a population. Studies at the Oak Ridge Na- tional Laboratory by the Russells and their coworkers, especially E. F. Oakberg, now clarify certain aspects of this situation. Female guinea pigs and hamsters, as well as monkeys, are much moreresistant than female mice to the sterilizing effect of radiation. It seems that in the female mice the oocytes in the ovary