- Abstract In this present study we have characterized the array of hemopoietic cytokines generated by fibroblasts in response to inflammatory signals. It was shown that murine embryo fibroblasts (MEF) are able to generate colony stimulating factors (CSFs) [granulocyte-macrophage (GM-CSF), macrophage-CSF (CSF-1) and granulocyte-CSF (G-CSF)] as well as the hemopoietin interleukin 6 (IL-6), while the production of IL-3, IL-4 or tumor necrosis factor (TNF) could not be detected in MEF, as assessed by bioassays or expression of specific mRNA. The production of colony promoting activity was observed when fibroblasts were stimulated by lipopolysaccharide (LPS) or individual cytokines [IL-1, interferon-γ (IFN-γ), IL-2, IL-4 or TNF] in serum-free conditions as well as by serum itself. These inducers differentially stimulated in MEF the production of various CSFs; LPS induced mainly CSF-1, while cytokines or serum induced equivalent amounts of GM-CSF and CSF-1. The production of IL-6 was induced by LPS in serum-free conditions, while stimulation by cytokines (IL-1 or IFN-γ) resulted in IL-6 production only in serum-supplemented cultures. Serum by itself did not induce IL-6 production by MEF. The secretion of IL-6 by fibroblasts was detected early and peacked after 6 hours, while CSF activity peacked after 24–72 hours, depending on the inducer. Constitutive mRNA expression of CSF-1 was detected in serum-free conditions in unstimulated MEF, however colony-promoting activity was detected only upon stimulation with cytokines, LPS or serum. Transcription of specific mRNA without detectable biological activity was also observed for IL-6 in MEF stimulated with IL-1 in serum-free conditions. Thus, multiple control levels are responsible for the production of hemopoitic cytokines by fibroblasts that may result in the amplification and fine tuning of immune/inflammatory reactions and hemopoietic responses.