The influence of erythropoietin (Ep) and burst-promoting activity (BPA) on the relative rates of synthesis of fetal and adult hemoglobins (HbF and HbA) were investigated in clonal cell culture. In the case of crude Ep, a positive dose-response relationship between the concentration and the relative rate of HbF biosynthesis by circulating erythropoietic precursors was clearly observed. In contrast, increasing the concentration of purified Ep enhanced the synthesis of HbF and HbA equally. BPA from bone marrow conditioned media and T-cell conditioned media enhanced the relative rate of HbF biosynthesis and burst number at limiting serum concentrations (10%-15%). We, therefore, conclude that BPA is important in the modulation of HbF biosynthesis in culture.