To clarify the defective erythropoiesis in eight patients with Diamond-Blackfan anemia, we studied their bone marrow response in vitro to recombinant human interleukin-3 (IL-3) and recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF). In an erythropoietin-containing assay system, specimens from six of the eight patients yielded low numbers of erythroid colonies compared to cont...