Granulocyte Colony Stimulating Factor (G-CSF) is a 19.6 kDa glycoprotein which stimulates the growth of neutrophil granulocyte precursors at the myeloid progenitor cell level. Functionally, G-CSF is a cytokine and hormone produced by a number of different sources in the body which include monocytes, mesothelial cells, fibroblasts, and endothelial cells. An important clinical application of measuring G-CSF is in the treatment of transient phases of leukopenia following chemotherapy and/or radiotherapy. G-CSF can also act on neuronal cells as a neurotrophic factor. This property is currently under investigation for the development of treatment of neurological diseases such as cerebral ischemia.