146. By the use of the above relationship a fit of the experimental data could then be attempted with optimization of the various parameters to achieve the minimum square deviations. The experimental points from the fitted curve gave the following solution y = 0.00662 ef7*9199 (28) in which the functional relationship between A and D is given by the above expression (equation 27). Figure XI represents the best fit to the experi- mental data in Figure X and provides for each dose (in the abscissa) the percentage of life~shortening to be expected (on the ordinate) at the dose rates (in rad/day) specified at the various points along the fitted curve in the body of the graph. THT. The neutron data in Figure X deserve separate mention because for them the very characteristic trend of the x- and gamma-ray data in duration-oflife experiments has not been verified. On the contrary, the data available would point to a continuously increasing effect of life-span-shortening with~ out any inflection in the trend. The data are too few and too scattered to establish with certainty such a difference in shape. It may be speculated that the effectiveness of the neutron treatment in duration-of-life experiments is . so high compared with x- and gamma-ray that the bending of the curve is not . possible in view of the short life of the irradiated animals. But it should also be remarked that the experimental points in excess of about 60 per cent of life-shortening (where the curvature becomes apparent with the low-LET radiation) are only two of the same series and therefore insufficient to confirm a difference of shape. In any case, the point is of little practical signi- ficance because the doses involved are extremely high. 148. It is of great interest to examine the change of effectiveness observed in the mouse between the single acute and the extremely low dose-rate data. If the linear term with dose of the single acute exposure (about 5 per cent life lost for 100 rad) is divided by the slope of the curve for duration-of- life exposure obtained, for example, at a dose-rate of 1 rad/day (total dose about 700 rad) the loss of efficiency is by about a factor of 7. In the case of neutrons an estimate of efficiency requires that a function be fitted to the data in Figure X. If a linear rationship is interpolated only for the purpose of the present comparison, the equal effect ratios between acute and chronic neutron exposures at 50 per cent, 20 per cent and 10 per cent of life- shortening are, respectively, 0.6, 1.4 and 2.5,