The deposition of energy in bone marrow by photoelectrons emitted from bone salt irradiated with x rays is a complicating factor in the interpretation of RBE values. It is emphasized that the calculations neglect the effect of backscattered radiaiion. This effect is negligible for the case of ihe mouse suspended in air. A physical model has been constructed to represent the spatial relationships between mouse marrow and the bones enclosing it. By the use of this model detailed calculations have been made of the energy absorption in marrow cavities of various sizes and shapes during whole-body exposure to 250-kv x rays or cobalt-60 gamma rays. The calculations involve a double integration over all photon and electron energies concerned. The average energy absorption by the total marrow of the mouse subjected to whole-body irradiation has been calculated to be 98.6 ergs/cm3/r from 250-kv x rays and 94.0 ergs/cm3/ r from cobalt-60 gamma rays. The significance of this difference in absorbed energy in interpreting the RBE of the two radiations is discussed.