In the redistribution of cells through the division cycle between dose fractions, the sensitizing effect is greatest when small doses per fraction are used. Since nonproliferating tissues limit the dose in radiotherapy and will not be sensitized by redistribution, the more the dose is fractionated and the lower the dose per fraction, the better. However, regenerative response in the tumor could limit its radiocurability if overall treatment time were protracted. This could be circumvented if two or more fractions were given daily, allowing time for redistribution; the probability of irradiating cycling cells in sensitive phases would then be increased.