qualitative differences between control and irradiated animals. However, mali- gant neoplasms developed at an earlier age in irradiated dogs, thus accounting in large part for the life-span-shortening. 87. The above data were reanalysed by Walburg |W1]| by the method of Kaplan- Meier [K2] for competing causes of death, the analysis being limited to controls and to dogs exposed to 100 R, where sufficient numbers were available. For ages at death beyond 3000 days (an epidemic of canine distemper or a vitamin-E deficiency altered to some extent the pattern of early deaths) there was a significantly increased rate of mortality in the irradiated dogs with respect to normal animals, but this increase disappeared when the neoplastic deaths were exlcuded from the comparison. Thus, in Walburg's opinion, the data are in accordance with the view that all the radiation—induced shortening of life seen at relatively low doses can be explained by induction or acceleration of neoplasia. 6. 88. Other mammals It is known that experiments on life-span-shortening in large animals were also carried out. In the burroirradiations with single and fractionated doses of gamma rays and with single doses of neutron-gamma radiation from the detonation of a nuclear weapon have been performed [B10]. Results on this se- ries are not sufficiently advanced for any definite conclusion. 89, In the cow, single and fractionated doses of gamma rays were also admin- istered in April 1960 for a life-span study [N5] but the experiment was terminated in 1973. The relevant data are of no use for life-span-shortening since more than half of the animals were still alive when the experiment was ended. T. 90. Conclusions Most of the data pertaining to the effects of single acute doses of x and gamma rays in the mouse are summarized in Table 1 and are plotted together in Figure II which shows the percentage of life-span-shortening as a function of dose. The data in the figurerefer to about 35 experimental series per- formed on about 20 strains of inbred, outbred of hybrid mice of both sexes and various ages, performed in various laboratories around the world since 1956.