Conditioned media (CM) of phyto-hemagglutinin-stimulated lymphocytes from chronic uremic patients were studied for burst promoting activity (BPA), using methylcellulose cultures with cord blood cells as a target population. Renal transplantation procedure was followed by a prompt rise of BPA, as well as the number of blood burst-forming unit erythroid (BFU-E) and hemoglobin levels, while no change in BPA and blood BFU-E number was observed in patients receiving rEpo and recovering from anemia. Thus, low BPA secretion from blood cells may have a role in the development of anemia in uremic patients.