Dose: cure-rate curves for tumours have been calculated from dose: survival curves for single cells, on the assumption that if a tumour is to be cured, every cell in it must be deprived of the ability to continue to reproduce. Host factors have been neglected.A value for the radiosensitivity of the cells in squamous carcinomata of the mouth and skin has been deduced from a dose: cure-rate relation for these tumours. A figure of 134 rads was obtained for the D0value of the cells; this is similar to the values which have been reported for a number of kinds of mammalian and human cells irradiated in culture and for a strain of leukemic mouse cells irradiatedin vivo.