Macrophage colony-stimulating factor (M-CSF) was found to be a glycoprotein with a molecular weight of 85 kDa which stimulated macrophage colony formation of mouse bone marrow cells in a semisolid agar culture system in 1978. M-CSF stimulates differentiation of progenitor cells to mature monocytes, and prolongs the survival of monocytes. It enhances expression of differentiation antigens and stimulates chemotactic, phagocytic and the killing activities of monocytes. Macrophage CSF also stimulates production of several cytokines such as granulocyte-macrophage CSF, granulocyte CSF and interleukin (IL)-6 by priming monocytes, and directly stimulates production and secretion of IL-8 and reactive nitrogen intermediates. In addition to the stimulation of hematopoiesis, M-CSF also stimulates differentiation and proliferation of osteoclast progenitor cells and cytotrophoblasts. Proteoglycan type M-CSF, which contains chondroitin sulfate chains, was found in 1992. In a large-scale double-blind controlled study on acute myeloid leukemia (AML), it has been shown that the administration of M-CSF to patients after consolidation chemotherapies shortens the periods of neutropenia and thrombopenia after chemotherapy and reduces the incidence and shortens the duration of febrile neutropenia, as well as shortening the period required to finish three courses of intensive consolidation therapy.