Because leukemia inhibitory factor (LIF) has little or no effect on murine hematopoietic progenitor cell growth yet enhances hematopoiesis in vivo, we sought to determine whether the effects of LIF were directly or indirectly mediated, or a combination of both. Although LIF alone or in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3) has no effect on colony formation of unfractionated bone marrow cells (BMCs), it enhances M-CSF- induced colony formation. In comparison, LIF synergizes with IL-3, GM- CSF, M-CSF, and Steel Factor (SLF) to promote the colony formation of partially purified lineage-negative (Lin-) BM progenitors without altering their differentiation. These effects were directly mediated since identical results were observed in single-cell assays. Comparing the effect of LIF with other members of this subclass of hematopoietins (IL-6, oncostatin M [OSM], and ciliary neurotrophic factor [CNTF]), we found that while LIF and IL-6 equally synergize with M-CSF and SLF to promote the colony formation of Lin- BMCs, OSM, and CNTF have no effect. In agreement with OSMs ability to directly bind gp130, preincubation of BMCs with OSM inhibits progenitor cell growth stimulated by the combination of LIF or IL-6 plus SLF. LIF can also directly enhance the growth of further purified more primitive Lin- c- kit+ progenitor cells in the presence of IL-3, GM-CSF, or SLF. Thus, LIF can directly synergize with growth factors to promote the proliferation of purified hematopoietic progenitors, suggesting that the direct effects of LIF on hematopoietic cell growth can, in part, explain the observed hematopoietic effects in vivo. This is a US government work. There are no restrictions on its use.