The existence of thrombopoietin, a hormone that controls blood platelet production, was proposed almost 30 years ago, but it has been only during the past 10-15 years that sufficient knowledge was obtained to establish that this humoral factor is a major in vivo controlling factor of thrombocytopoiesis. This concise review attempts to summarize the hormone's history, background, effects on megakaryocytes and platelet production, sources, and purification and characterization, along with the current status of research on this hematopoietic-stimulating factor. New evidence for biological and chemical similarities of thrombopoietin from thrombocytopenic animals and human embryonic kidney cells is also presented.