338 RADIATION STANDARDS, INCLUDING FALLOUT locus per 100,000 gametes—the curve at low dose rate runs up to 6 mutations per locus per 100,000 gametes. They have not tested it to quite as high a dose. In the female sex they have only really tested, or at least published, on two doses, one of 400 roentgens at the high dose rate, and one of 250 roentgens at the low doserate, so that the difference between the effect of a high dose rate and the low dose rate in the female germ cells is apparently greater than it is in the male reproductive cells. The data on the male germ cells are much more extensive. For both sexes taken together the yield at the high dose rate is about six times as great as at the low dose rate. The most recent experiments, of which I have received a report just over the past weekend from Dr. Russell, have refined the detinition of “high dose rate” and “low dose rate” and reveal a situation of increasing complexity. Initially 90 roentgens per minute and 0.009 roentgen per minute were used for the high and lowdose rates, respectively. The quality of the radiation used was also different, 250-kilovolt X-rays for the high dose experiments and cobalt 60 gammaradiation for the low dose rate. But in experiments in which a high dose rate and low dose rate were both administered from the same source, any significant difference owing to the quality of the rays was ruled out. he new tests have been conducted at intermediate dose rates of 9 roentgens per minute and about 0.8 roentgen per minute. These prove to be in the right critical range. For the male germ cells, the mutation rate at 9 roentgens per minute is intermediate between the results at high and low dose rates. At 0.8 roentgen per minute the mutation frequency is already that characteristic for a low dose rate. In the case of the female germ cells, the situation appears to be somewhat different. The 0.8 roentgen per minute rate still yields an intermediate frequency of mutations. Obviously, the situation is quite complicated and a good deal more study will need to be doneto clarify it completely. I would hke to emphasize that in terms of human exposure0.8 roentgen per minute is not exactly what one would consider a very low dose rate, since at that rate it would require only 12 to 13 minutes to equal the level which was set by the NAS Genetics Committee as an upper limit for the average gonadal dose. Work to confirm these results of Russell and his group has not gone very far at the present time. Some work has been done with Drosophila, but the results are conflicting. There is, I am told, a study at Harwell in England on mice which has given preliminary results that confirm the effect for the mice. There isalso one at Harwell conducted for the fruit flies which confirms the dose rate effect. In the mouse it is impossible without vast expense and labortotest. the mutation rate for doses much lower than 86 roentgens. Westill must resort to the fruit fly for study of very low doses. In my own laboratory we have recently completed a 3-year study of the mutation frequency produced by a dose of only 5 roentgens to the mature male and female germscells, whichis, I believe, the lowest. dose studied for its mutagenic effect in any animal up to this time. Dominant mutations of a particular minute bristle type were studied, and a total of 1,360,948 individual flies descended from parents which had received a 5-roentgen dose of X-rays were scored. We called this a megafly