the incidence of diseases in the population examined. Similarly, the LD. 4/39 test showed that resistance followed the same dependence on age as after the protracted exposure test. The conclusion was that the earlier onset in the decline of resistance of the more heavily exposed animals was in fact corre- lated with the earlier onset of morbility in these groups. eTh. In addition to presenting data on CBA female mice given acute doses of radiation (450 rad of 250 kVp x rays at four ages from 100 to 670 days), Mole [M2] pointed out some difficulties in the analysis of such data. If the mean after-survival time is taken as the criterion of effect, then it is clear that the radiosensitivity of the animals decreases with age. But if it is assumed that the radiation-induced mortality is not independent of the mortality of the non-irradiated animals and the Abbott's correction is used to derive the net radiation-induced life-shortening, then the curves of cumulative mortality show little difference with age. Mole refers also to similar unpublished data obtained with protracted exposures to gamma rays and fission neutrons. Such observations might imply that the life-shortening process proceeds independently of natural aging and thus’ the two phenomena are not correlated. However, in the absence of further information about natural aging itself, it should simply be realized how opposing conclusions might be reached by different analyses of the same experimental data. 275. In experiments by Yuhas [Y3] the sensitivity was studied of 4 to 2h months old C57BL/6J female mice to the life-shortening effects of 1400 R of 300 kVp x rays. Radiation was given in 10 equal fractions within 12 days. In the 4 months old animals the resulting life-shortening amounted to 148 days but in older animals the same dose was considerably less efficient: in fact, life-shortening amounted to only 30 days at 9 months and at the three oldest ages (15, 18 and 24 months) there was actually a lenghtening of life of the order of 53 to 65 days. 276. The decreasing response with increasing age confirmed previously reported data by Lindop and Rotblat [L12], Kohn and Guttman [K6], Jones and Kimeldorf [J3]. The data also confirmed the life-lenghtening in mice irradiated at very old ages. The compatibility of these findings was tested in relation to five different hypothesis. The data could not be accounted for in terms of insuf- ficient time for expression of injury; or of the identity of normal and "