Under normal conditions the circulating platelet mass is remarkably constant, which suggested early on that a negative feedback mechanism was involved in the control of thrombocytopoiesis. Furthermore, plasma or serum from thrombocytopenic animals or humans has been reported to stimulate platelet production when injected into laboratory animals, thus suggesting that it contained a thrombopoietin(s). During the past decade, the biochemistry and physiology of thrombopoietin have been studied. However, it is not now well established that thrombopoietin is a glycoprotein produced, at least in part, by the kidney. Moreover, particular attention may be directed to its machanism of action.