of normal. Only the results with monoenergetic-, fission spectrum- and wea- pon-neutrons delivered acutely were included, for a total of 15 series and 5 different strains of mice. Inspection of the data makes it immediately ap- parent that the nature of the dose-relationship is in this case quite different from that observed with x- and gamma-rays. This impression is confirmed when the following equations are fitted to the data with the following results y = a+bD Ro = e y=atbDtiecD y = 5.7361 + 0.0851 D = 0.5751 (10) y -= 2.005 + 0.1837 D - 277 10° -) D 2 FO 2 = 0.6860 (9) . (11) (12) Clearly, none of these two relationships provides a satisfactory interpolation to the neutron data, because the first of them fails to show the initial steep rise and the latter, after showing a maximum of effect between 300 and 400 rad bends down rapidly towards lower values, an effect which would be difficult to interpret. The following relationship was also fitted y = atbvbd (13) and it yielded the following solution y 0.1645 + 1.764 VD = Re (14) 0.6837 The square-root relationship seemed to fit the data fairly well in that it described adequately the increase of effect seen at very low doses of neutrons and the ensuring levelling-off of the data for doses up to 500 rad, along a slope roughly parallel to the slope of the low-LET radiation dose relationship. 95. Thus, whatever the actual relationship truly applying to the life-span- shortening effect caused by neutrons in the mouse, under the conditions of the present analysis the data are best described by a relationship having a convex upward trend with dose, such that the efficiency of low neutron doses is higher than that of higher doses. The numerical value of this higher effi-