Recently we have published a new model of survival cellular response to radiation based on the existence of a saturable cellular repair mechanism. In the present paper we extend the predictions of this model of calculating the change in total dose necessary to achieve an equal response in a tissue when the dose per fraction in a radiotherapy fractionation schedule is varied. The model provides a new explanation of the difference between late and early radiation reactions. Results obtained from the model for different tissues and standard fractionation schedules are approximately equal to those obtained by the L.Q. (linear-quadratic) model. The model is compatible with in vitro survival curves that are straight at high doses.