Summary
Telophase abnormalities in Ehrlich ascites-tumour cells were observed 18 hours after the cells had been irradiated either with 1·5 mev x-rays or fast neutrons. The sensitivity of well-oxygenated tumour was greater than that of anoxic tumour by a factor of 2·5 in the case of x-rays, but of only 1·3 in the case of neutrons. After neutron-irradiation the abnormalities showed an exponential relationship to dose, but after x-irradiation the abnormalities increased more rapidly than the first power of the dose. When the proportion of abnormalities was 50 per cent the RBE in O2 was 3·2, but in N2 it was 6·5.