110. The survival of two mouse strains (the ABC male and the CF1 female) was studied by Sacher |S2] for exposure rates between 2 to 100 R/day of x rays. The relationships between mean survival time and mean accumulated dose as a function of exposure rate, as well as the relationships between lethality following acute or repeated exposures were analysed. From these analyses a leth- ality function could be deduced under certain mediating assumptions about the linearity of the processes and about a defined lethal bound of injury. 111. Neary, Munson and Mole 'N3] exposed male and female CBA mice to gamma rays (1.5 mR/h) or fast neutrons (three dose levels; 36.4, 3.0 and 0.9 mrad/h) spread over most of the time and almost continuously. Gross effect on the cu- mulative mortality curve were seen only at the highest dose-rate where the mean survival time was shortened by one third; at the medium dose-rate the mean survival times was shortened by less than 10 per cent. There was a general trend towards shorter survival with increasing dose levels: the difference of the combined data for males and females was significant at the 5 per cent level only between the group at the highest dose level and all other groups and between the control and the group at medium level. Significance could be improved if only the animals comprised between 30 per cent and 100 per cent mortality were included in the analysis. Shortening of mean survival time was examined in re- lation to dose-rate and to mean accumulated dose after averaging the male and female data. Having made allowance for the gamma ray contamination of the neu- tron field, straight lines could be drawn through the experimental points. 112. Neary et al. [N3] also analysed the data to see whether treatment was acting by accelerating the process of aging, in which case the time axis of the mortality curve would be contracted and the dispersion of life-time values would be reduced in proportion to the mean life-time. The evidence was, on the contrary, for a displacement to the left of the time axis which would be in favour, as in the case of Lindop and Rotblat [L1]|, of premature rather than ac- celerated aging. 113. All experiments on irradiation for the duration of life known until that time were reviewed by Mole in 1957 [M13] and his paper was made a part of the 1958 report of the Committee [U1]. Only 5 out of 11 known reports (see [M13] for a complete list of references) contained sufficient details for the reconstruction of a curve showing the decrease of the mean survival time versus the dose/week. Fast-neutron as well as gamma ray data were plotted together