2. e0T. Conclusions Exeept for a study of thermal column radiation MeV protons [S17] and another on 60 [C12] - for which, as expected, the RBE was found to be near unity - experience on life-shortening refers only to neutron irradiations. Fission and degraded-fission spectrum neutrons, weapon and monoenergetic neutrons were used in the various cases. The only species on which data are available is the mouse, where inferences may be drawn about the RBE changes as a function of dose fractionation and protraction and, to some extent, as a function of the dose level. 208. For acute doses in the region of a few tens to a few hundred rad of x and gamma rays (and equivalent doses of neutrons) RBE figures of 2 to 3 have generally been found [U5, S18, S44]. Direct comparisons of single and brief daily courses of irradiation were reported by Vogel, Frigerio and Jordan [V1] and by Vogel and Jordan [V3, V4, V5, V6] at neutron dose rates of between 1 and 35 rad/ min: under these conditions the RBE is very similar to that applying to acute single doses (2.8) with maximum oscillations between 2 and 5. These figures are also similar to those of Upton, Randolph and Darden [U10! (2.7 to 5.4) obtained by changing the instantaneous dose-rate form 7 rad/min to 5 rad/day, for total doses of up to 600 rad. The data of Curtis and Gebhard [C8, C17] are in this respect an exception in that the RBE is similar for single and for frac- tionated doses and, on the whole, very low (1.7). The evidence reviewed shows some increase of the RBE upon short fractionation courses and with decreasing dose-rate down to a few rad of neutrons per day, from figures of 2 - 3 applying to acute treatments to figures of about 5 - 6. More consistent changes are found for extremely long courses of irradiation. RBE values of between 9 and 15 were quoted under similar conditions by Neary, Munson and Mole [N3!/ and Neary, Hulse and Mole [N8]. These latter are in fair agreement with the figure of 13 that may be obtained from the 1958 report of the Committee [U1]. 209. For more precise evaluations the data of Upton, Randolph and Conklin [U7] are of particular interest because they examine the RBE changes as a function of the dose-rate, with a spectrum of dosages between the acute and the long chronic exposures. The RBE changes in this series from figures of about 3 at high dose rates to figures of about 8 for protracted exposures or even up to about 14 for very prolonged treatments. Ainsworth et al. [A7! also reported differences in the RBE of fission neutrons between 2.0 - 2.4 up to 10 - 15 for weekly dose fractionation over 23 weeks and total doses a few hundred rad of gamma rays and a few tens of rad of fast neutrons.