The proliferation and function of hemopoietic cells derived from genetically anemic Sl/Sld mice have been studied by the use of cell transplantation technics. It was found that marrow cells derived from anemic Sl/Sld or Sld/Sld mice, when implanted into heavily irradiated mice of genotype +sl/+sl, are capable of forming macroscopic spleen colonies, with approximately the same frequency as cells derived from normal +sl/+sl mice. Marrow cells derived from Sl/Sld animals were tested for their capacity to cure the anemia of W/Wr mice and were found to implant as rapidly and to have as long-lasting a beneficial effect as did marrow cells from +sl/+sl mice. The radiosensitivity of the colony-forming ability of marrow cells from Sl/Sld mice was found to be similar to that found previously for +sl/+sl marrow cells, although the anemic animals are known to be more sensitive to total-body irradiation than are their normal littermates. Marrow cells from +sl/+sl mice were found to proliferate more slowly in irradiated mice of genotype Sl/Sld than in irradiated +sl/+sl littermates, even when the anemic and the normal mice were joined in parabiosis. These observations indicate that the hemopoietic colony-forming cells in mice of genotype Sl/Sld are normal, but fail to function adequately because the tissues of mice of this genotype are unable to provide sufficient support for proliferation and differentiation of these progenitor cells.