Marrow obtained from mice (referred to as [X + BM] mice) 3 months after gamma-irradiation (9 Gy) and bone marrow inoculation (0.1 femur equivalents) showed a reduced capacity to reconstitute hemopoiesis of irradiated mice and an increased sensitivity to 5-fluorouracil. Sorting of marrow from (X + BM) mice on the basis of low angle and 90 degrees scatter, and low rhodamine 123 fluorescence, showed that the set of cells that in normal mice is enriched for cells efficient at hemopoietic reconstitution manifested the greatest reduction in hemopoietic reconstituting ability. In spite of this reduction this fraction contained as many 13-day spleen colony-forming units (CFU-S13) and high proliferative potential colony-forming cells (HPP-CFC) as the equivalent fraction from normal littermate mice. This could be explained by postulating that neither CFU-S13 nor HPP-CFC are responsible for hemopoietic reconstitution, but that this is dependent on an earlier, pre-CFU-S13 cell. Alternatively only a subset of either CFU-S13 or HPP-CFC is responsible for long-term hemopoietic reconstitution after lethal irradiation. It would appear that at present there is no adequate method of predicting the hemopoietic reconstituting ability of a given marrow, other than to test it by injection into lethally irradiated hosts.