The burst formation from human and murine burst forming unit-erythroid (BFU-E) requires the presence of erythropoietin (Epo) in semi-solid cultures of bone marrow cells. A number of haematopoietic factors are described that increase the burst number: interleukin 3 (IL-3), stem cell factor (SCF), granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-9, IL-11, insulin-like growth factor I, and erythroid potentiating activity (EPA). The authors now show that another activity present in medium conditioned from adult or fetal human kidney cells specifically stimulates the proliferation of BFU-E. A cell line derived from fetal kidney produced such an activity, which was shown to be different from the previously cited haematopoietins, acted on CD34(+)-enriched BFU-E and promoted an increase in CFU-E number in the bone marrow of injected animals, could be precipitated using 40% ammonium sulfate, was destroyed by proteolytic enzymes and was shown to be a glycoprotein by its retention on ConA-Sepharose. The authors propose to call this apparently novel activity, which influences only the number of bursts, human erythroid burst-stimulating activity (hEBSA).