Abstract— Protection of X‐irradiated mice by bovine superoxide dismutase is enhanced when the enzyme is given intravenously both before and after the exposure. With the combined treatment, the LD50(30) dose is increased from 734 ± 8 to 1144 ± 15rad for a dose reduction factor of 1.56 ± 0.04. This protection occurs in a dose range where hematological damage is an important contributor to animal lethality. The proliferative capacity of bone marrow stem cells, X‐irradiated in air, is protected by exogenous superoxide dismutase. The enzyme increased the D0 from 105 ± 6 to 290 ± 34rad, an increase that represents 83% of the oxygen enhancement ratio of 3.3. In N2 and N2O, the D0 of the stem cells is 348 ± 50 and 327 ± 55 rad, respectively, and the enzyme does not significantly change these values.