Macrophage Colony-Stimulating Factor (M-CSF), which is permanently present in blood and human bone marrow, regulates the proliferation, differentiation and functions of cells of the mononuclear-phagocytic lineage. By using Reverse-Transcriptase Polymerase Chain Reaction (RT-PCR) we demonstrate that human marrow stromal cells express two types of M-CSF transcripts that are translated into the secreted form and the membrane anchored form. By using a specific and sensitive ELISA, we found that the spontaneous production of M-CSF by human marrow stromal cells is enhanced after stimulation with lipopolysaccharide (LPS), phorbol myristic acetate (PMA) and most interestingly by the lipidic mediator of inflammation platelet-activating factor (PAF). Thus, marrow stromal cells might represent a regulated cell source of bone marrow-derived M-CSF. These results not only emphasize the importance of the bone marrow environment in the control of human hematopoiesis but also evidence, for the first time, the potential role of PAF in the marrow cytokine network during inflammatory processes.