The mechanisms of radiation damage production are used to examine the following premises: (1) the number of DNA double-strand breaks per unit dose increases with dose; (2) cell type to cell type variations in yield of DNA dsb per dose occur. Two stages of damage production are identified as possible sources of damage yield modulation; numbers of OH. free radicals reacting with the target, and amount of chemical repair occurring on the target radicals. These factors are discussed in the light of the structures within which cellular DNA is packaged and the known rate constants for the reactions involved. It is concluded from our current knowledge that, in the presence of oxygen: (a) the number of DNA dsb is linearly related to dose, and (b) the yields of DNA damage per dose among cell types are constant. There is a caveat to the latter conclusion: the chromatin structure may be different in radiosensitive cell lines. In the absence of such a difference, variations in radiosensitivity with dose or with cell type are assigned to differences in repair speed and/or accuracy.