Purified recombinant granulocyte colony stimulating factor (G-CSF) and interleukin-6 (IL-6) stimulated the formation of similar numbers of colonies in cultures of normal mouse marrow cells. LIF and IL-6 induced comparable differentiation in clonal cultures of murine M1 leukemic cells and exhibited enhanced actions in combination. However, LIF was 16-25-fold more active than IL-6. Induction of differentiation in M1 leukemic colonies by both LIF and IL-6 was enhanced by the addition of G-CSF or M-CSF but not by GM-CSF or Multi-CSF. Both G-CSF and IL-6, but not LIF, were able to induce differentiation in murine WEHI-3B leukemic colonies, but G-CSF was 10-fold more efficient than IL-6. Both G-CSF and IL-6 were able to stimulate the proliferation of cells of the NFS-60 continuous cell line, but G-CSF was 30-fold more efficient. M1 cells constitutively produced low levels of IL-6 and production was enhanced by LIF, but the general characteristics of the actions of LIF, IL-6, and G-CSF suggested that each operates independently as a direct differentiation inducer of leukemic cells. The similarities in the biology and actions of G-CSF, LIF, and IL-6 suggest that they may be designed to exhibit coordinated biological functions in certain situations.