Table 3 Approximate scales of sensitivity of various animal species for the life-shortening effects of irradiation Irradiation condition Approximate sensitivity more more sensitive Chronic x-irradiation monkey resistant rat, dog and mouse about equal Acute x= or Refs. rabbit [B11] rat and mouse fu 1] guinea-pig, rat, dog and mouse about equal [G 1] [$23] man, dog and mouse about equal [G 6] Chronic y-irradiation dog and mouse IN 7] Chronic y-irradiation goat mouse [fH 9] Y-irradiation Chronic y-irradiation Chronic y-irradiation about equal man guinea-— pig about equal they refer to the SAS/4 strain [L1, L2], to the CFW strain [M14], to the RF/Un strain [U7] and to the B6CF1 strain [AT]. The data in the LAF1 mouse are con- tradictory since in the series by Upton et al. [U5] the female appeared to be more sensitive, in that by Sacher and Grahn [S4] the females were more resis-— tant at survival times of 130 days or more. In the case of the CAF1 mouse [K6] age at irradiation and sex appeared to interact, owing probably to changes in the endocrine balance, so that in early life the female and in old age the male appeared as the most sensitive sex. In all other cases comprising the follow- ing inbred strains: CBA, Balb/c, A/Jax, A/He, C3CF1, B6RFMF1, C3CBORFM; invaribly a higher sensitivity of the female animals was observed. factors reported were between 1.5 and 4. The sensitivity Ovarian disfunction induced by irra- diation and the incidence of ovarian tumours were reported to cause this differential effect, which tended to disappear when the data were appropriately corrected for the incidence of tumours of the genital tract or of leukaemia. in the case of the CAF1 mouse Only [K6] the ovarian tumours were reported to be un- correlated with the higher sensitivity of the female and in that instance it appeared that irradiation in late life did actually show a higher sensitivity of the males. The observation therefore does not contradict the conclusion to be