The in vitro actions of leukemia inhibitory factor (LIF) purified from Krebs tumor conditioned medium, were analyzed on murine leukemic M1 and WEHI-3B D+ cells and on normal hemopoietic progenitor cells. LIF has no observable effects on WEHI-3B D+ cells but rapidly induced macrophage differentiation and loss of clonogenicity in M1 cells, resulting in the formation of abortive clones or differentiating colonies of reduced size and number. These effects were observable within one to two cell divisions in the presence of LIF and were irreversible. Addition of macrophage-colony-stimulating factor (CSF) but not granulocyte/macrophage-CSF, granulocyte-CSF, or multi-CSF reduced the LIF-induced suppression of colony numbers and size. G-CSF had a slower differentiation-inducing action on M1 cells than LIF but potentiated the differentiation-inducing effects of low concentrations of LIF. LIF had no colony-stimulating activity for normal granulocyte-macrophage progenitor cells and did not alter their quantitative responsiveness to CSF. However, culture of normal progenitor cells in the presence of LIF, but initial absence of CSF, reduced the survival of these cells. The differing actions of LIF and G-CSF on M1 leukemic cells suggest the existence of distinct mechanisms for inducing macrophage differentiation in these leukemic cells.