167. Ainsworth et al. [AT] reported that fractionation of a gamma dose of 838 rad into 2h doses of 35 rad administered over 23 weeks produced a sparing effect by approximately three-fold. On the contrary, a similar regime of frac- tionation with fission spectrum neutrons produced an increased life-shortening, which is a rather unusual observation in fractionation experiments. Histo- pathological observations on pulmonary tumours could not explain all of the increased mortality resulting from neutron dose fractionation. Tentative ex- planations were offered, based on the differential acceleration of the lung tumour appearance or on the differential killing of potentially transformed cells in the lung. 168. Storer et al. [S44] exposed RFM and Balb/c mice to 7 weekly doses of fission neutrons of 6.7 rad for 7 weeks (total of 47 rad) or to 23.5 rad once every 4 weeks over 28 weeks (total 188 rad). Animals exposed to 47 rad frac- tionated had a life-span not different from those given single high dose-rate exposures or exposures at 1 rad/day. Balb/c animals receiving 188 rad had a survival time significantly shorter than that following a single exposure. RIM animals given fractionated treatment up to 188 rad experienced the same survival as after single exposure and a significantly longer survival than following exposure at 1 rad/day. It was concluded that the cause of death may cri- tically determine the effectiveness of the fractionated exposure, although the authors were unable to provide a basis for their observations. 169. Data on the effect of dose fractionation on life-~shortening were de- scribed in the Wistar male rat by Hursh et al. [H5| irradiated with x rays. While the life-shortening caused by an acute exposure of 600 R was marked (19 per cent of the control) animals receiving 20 R x 30 days had a life-span similar to that of the controls. Sixty R x 10 days produced an intermediate ef- fect between the previous two conditions. A pneumonia infection occurring about one year after the start of the experiment complicated the survival picture of the animals. Also, since the range of control values in these experi- ments was extremely variable and different from previous data, all this series seems rather inconsistent. 170. Data on x-ray dose fractionation were reported in the same animal spe- cies by Lamson, Billings and Gambino [L4, L11]. Increasing the number of fractions from 1 to 3 to 6 for the same total exposures of 120, 240, 480 R caused an increase of life-span, compared to the same exposure in one frac-