Abstract Macrophage colony-stimulating factor (M-CSF) stimulates the survival, proliferation, and differentiation of mononuclear phagocytes. In this study, the qualitative and relative quantitative ability of various cytokines to induce and to synergize in M-CSF production by monocytes was studied. GM-CSF and the phorbolester PMA were strong inducers of M-CSF m-RNA expression. This was correlated closely with the secretion of M-CSF protein as measured in the murine M-NFS-60 cell line bioassay. Both TNFα and IFN-γ enhanced M-CSF message levels induced by GM-CSF, but only TNFα synergized with GM-CSF in the induction of M-CSF protein secretion. M-CSF transcripts induced by TNFα and IFN-γ were much lower compared to those induced by GM-CSF and PMA and were not accompanied by the secretion of M-CSF protein. In addition, costimulation of cells with TNFa and IFN-γ did not result in M-CSF production. Although M-CSF did not induce its own message, it further enhanced M-CSF transcripts induced by GM-CSF. LPS also failed to induce M-CSF message or secretion. These results show that cytokines differ in their ability to induce or to synergize in the induction of biologically active M-CSF protein. They further demonstrate that M-CSF message expression, induced by cytokines, does not always correlate with M-CSF protein secretion.