5 per cent level than that of the males. When only the mean survival times between 30 per cent and 100 per cent mortality were considered in order to ex- clude early mortality, this sex difference remained evident. PUT. Sex differences in Balb/c, A/Jax, A/He, C3Hf/He and C57BL/6 mice were evident when data for female or for male animals of all strains were pooled together as a function of dose and fitted by linear regressions: the reduction of mean after survival for a given dose increment was about twice in female than in male animals [G10]. Ovarian disfunction resulting in ovarian tumour forma- tion was suggested as the possible cause of this difference. Grahn and Sacher [G3] also showed a greater sensitivity of the females with respect to chronic Life-shortening injury. With x rays of three different energies the reduction of life-span averaged in animals surviving the LD female and 29 per cent in the male sex. about 37 per cent in the Female nies were uniformly more sensi- tive than males when irradiated with gamma rays or fast neutrons [U5]. This was again attributed to the induction of tumours of the ovary or to hormonal disturbances. However, the RBE for neutrons against gamma rays was not signi- ficantly different between the two sexes. oe. After single-dose irradiation of one hybrid and five inbred strains of mice (200 kVp x rays, doses between 0.85 and 1.15 times the LD differences within sex were not significant. ) strain The two sexes had, aoever, sig- nificantly different responses averaging for the males 0.28 days lost/R and for the females 0.81 days/R (all strains pooled). Difference tended to disappear when corrections were introduced for animals dying of leukaemia and of ovarian tumours. Life-shortening in all strains after an acute treatment about the 150/30 seemed to be characterized by a single parameter applicable to all strains and sexes and expressed as a constant number, 28 days of life lost/100 R. Com- bination of this primary injury term with other secondary parameters could pro- vide equations predicting the final long-term effect for any combination of normal life expectancy, leukaemia incidence, ovarian tumour incidence and dose [G4]. 2h9. In the experiments of Lindop and Rotblat [L1] the shapes of the survival curves were similar for males and females SAS/ mice but showed small consistent differences in favour of a higher resistance for males. These differences were not such, however, to warrant a separate analysis of the life-shortening effect in the two sexes. Moos [M14] also reported no difference in longevity response