G-CSF is secreted by monocytes, macrophages, and neutrophils after cell activation. It is produced also by stromal cells, fibroblasts, endothelial cells, epithelial carcinomas, acute myeloid leukemia cells, and various tumor cell lines. The synthesis of G-CSF can be induced by bacterial endotoxins, TNF, IL1 and GM-CSF. Comparison of the primary sequence of G-CSF with those of the two other colony stimulating factors, GM-CSF and M-CSF, shows that the three factors are not related to each other. G-CSF stimulates the proliferation and differentiation of hematopoietic progenitor cells committed to the neutrophils and granulocytes lineage in a dose-dependent manner. G-CSF synergises with some other cytokines, including GM-CSF and IL4.