The effect of low doses of 240 kVp X rays or of 3 MeV neutrons has been investigated using skin reactions on mouse feet as the biological system. Eight or nine repeated small doses of radiation were used, followed by graded "top-up" doses to bring the reactions into a detectable range. By comparing dose-response curves, the RBE has been determined for neutron doses per fraction ranging from 0.25-1.0 Gy. The data are consistent with a limiting RBE of between 7 and 10 at very low doses. A review of other published RBE values for low doses per fraction shows a wide range of RBEs . Very few studies show a plateau value for the RBE. These findings are more consistent with dose-response data that fit a linear-quadratic model than with a multitarget single-hit model.