Experiments measuring the sensitivity of mouse bone marrow stem cells (CFU-S) and granulopoietic progenitor cells (CFU-C) to γ-rays and neutrons are described. Both populations are more sensitive to neutrons than to γ-rays, and in each case CFU-S are more sensitive than CFU-C. The CFU-C (but not CFU-S) show a biphasic dose-response curve to γ-rays, and the data suggest that 20–50% of CFU-C in normal mice are more resistant to γ-rays than the rest, possibly due to hypoxia. This difference between the two cell populations may be related to differences in spatial location in the bone marrow. The recovery of these cells following γ-rays is consistent with previous data which suggests that CFU-C are not self-maintaining, but require to be fed in from the stem cells.