The effects of X ray and 14 MeV neutron irradiation on growing tibia of mice were studied. As reported in the literature, the age of the animal at irradiation was an important factor if the endpoint in analyzing the radiation effect is the absolute shortening, difference in length between unirradiated and irradiated tibia, achieved by the treatment. However, if the growth remaining at the moment of irradiation is taken into account no significant differences in effect were observed with aging, and the dose given appeared to be the overruling factor. This was valid for both X ray and neutron irradiation. Absorbed dose-response curves after X ray irradiation showed a shoulder in the lower range of absorbed doses, which was not the case for neutron irradiation. RBE values for 14 MeV neutrons as compared to 250 kV X rays varied from 3.6 to 2.0 depending on the single absorbed dose level. No differences in RBE were found with aging. A mathematical formula was derived which can predict the absolute shortening when the initial and final length of the tibia are known as well as the absorbed dose of irradiation to be administered.