M-CSF is a cytokine that controls the production, differentiation, and function of macrophages. The active form of the protein is found extracellularly as a disulfide-linked homodimer. M-CSF is released by fibroblasts, breast cancer cell lines, alveolar macrophages, stromal bone marrow cells, endothelial cells, and mesenchymal cells. It binds CD14+ monocytes and promotes the survival/proliferation of human peripheral blood monocytes. In addition, M-CSF enhances inducible monocyte functions including phagocytic activity, microbial killing, cytotoxicity for tumor cells as well as synthesis of inflammatory cytokines such as IL-1, TNF-α, and IFN-γ in monocytes. M-CSF induces RANKL production in mature human osteoclasts and, consequently, this makes M-CSF a potent stimulator of mature osteoclast resorbing activity. Also, M-CSF induces VEGF production in human monocytes and human tumor cells. High levels of M-CSF, mononuclear phagocytes, and VEGF are associated with poor prognosis in patients with cancer.