RANULOCYTE colony-stimulating factor (G-CSF) is G a polypeptide growth factor that regulates the production of neutrophilic granulocytes. This physiologic process serves as the foundation for critical host defense systems and occurs on a large scale in vivo. An adult of average size will produce approximately 120 billion granulocytes per day simply to replace normal losses. This enormous production capacity may be increased by at least 10-fold under stress conditions such as infection. G-CSF is likely to play a role in the basal regulation of neutrophil production as well as to function as a primary regulatory factor controlling the neutrophil response to inflammatory stimuli. Further, G-CSF exhibits other biologic activities besides proliferative effects: specifically, G-CSF appears to modulate certain neutrophil functions as well as the distribution of neutrophils and progenitor cells within the body. G-CSF possesses unique and interesting characteristics among the family of hematopoietic growth factors. This review will summarize the current state of knowledge of the structure and function of G-CSF and its receptor.