Immunoassays have recently made it possible to specifically measure the circulating levels of hematopoietic growth factors. This is helping us to understand the in vivo regulation of hematopoiesis under conditions of steady state or stress, providing insights into the physiological roles of hematopoietic growth factors and their importance in the pathogenesis of disease. As mediators of pathological processes, hematopoietic growth factors may be targets for antagonist therapy in some diseases. Exogenous hematopoietic growth factors may also be useful therapeutically to augment physiological responses, so understanding hematopoietic growth factor regulation and serum levels may assist the development of specific therapies. Hematopoietic growth factor levels may also serve as tumor markers and assist prognostication or monitoring during the clinical course of an illness. The growth factors of particular interest include the four classic colony-stimulating factors: granulocyte-macrophage colony-stimulating factor, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, and multi-colony-stimulating factor, also known as interleukin-3. Other cytokines with hematopoietic growth factor activity include interleukin-1, interleukin-6, interleukin-11, stem cell factor (also known as Steel factor or mast cell growth factor), and leukemia inhibitory factor.