We investigated the in vitro hematopoietic stimulatory activity of leukemia inhibitory factor/human interleukin for DA cells (LIF/HILDA) on bone marrow progenitor populations in 17 normal individuals. In serum-free cultures LIF/HILDA did not induce colony growth. In serum containing media, LIF/HILDA stimulated the growth of colony forming unit (CFU)-MIX and CFU-EO in a dose-dependent fashion and resulted in an increased CFU-MIX and burst forming unit-erythrocytes (BFU-E) colony size. Similar stimulatory effects were seen on a highly purified hematopoietic progenitor population obtained after immunomagnetic depletion of mature myeloid precursors and lymphoid cells. Addition of LIF/HILDA to cultures containing maximally stimulatory concentrations of recombinant human interleukin-3 (rhuIL3), rhuIL3 + rhuIL6, or rhu granulocyte-macrophage colony-stimulating factor (rhu GM-CSF) in serum containing media significantly increased the number of CFU-MIX and eosinophil colonies and increased size and cluster number of CFU-MIX and BFU-E. Depletion of accessory T lymphocytes or monocytes from bone marrow progenitors did not alter the response of hematopoietic precursors to LIF/HILDA. A similar increased colony growth was seen when LIF/HILDA was added to cultures of positively selected CD34/HLA-DR+ or CD34+/HLA-DR- bone marrow hematopoietic progenitor cells stimulated with maximally stimulatory concentrations of rhuIL3 + rhuIL6. LIF/HILDA is a novel cytokine capable of stimulating growth and proliferation of multi-lineage, erythroid, and eosinophil colonies in the presence of serum. LIF/HILDA exerts its activity by direct interaction with highly purified immature bone marrow progenitor cells, has an additive effect when used with other cytokines known to stimulate primitive hematopoietic precursors, and does not require accessory cells.