Erythropoietin is the only hematopoietic growth factor that behaves like a hormone. Produced in the kidneys and the liver, erythropoietin interacts with erythroid progenitor cells in the bone marrow to promote their proliferation and maintain their viability. Erythropoietin production is regulated at the level of its gene by tissue oxygenation; hypoxia or anemia stimulates erythropoietin production, and erythrocytosis suppresses it, but never completely. The plasma erythropoietin concentration reflects erythropoietin production and can be used to define erythropoietin-deficient states in which anemia may be amenable to correction by administration of recombinant human erythropoietin.