We cultured human umbilical cord blood and adult peripheral blood erythropoietic precursors in methylcellulose clonal assay and measured the synthetic rates of HbA, A2, and F. Hb was labeled with14C-amino acid in culture and separated by slab-gel isoelectric focusing and quantitated by autoradiography. While the mean percentage of HbF synthesized by adult cells was only 20.1%, that of umbilical cord blood cells was 53.9%, which corresponds closely to the biosynthetic capabilities of umbilical cord blood reticulocytes. Variations in the erythropoietin concentrations did not influence the percentage of HbF. Erythropoietic cell cultures of human umbilical cord blood may provide an important means for studying the molecular mechanisms controlling physiological Hb switching in the perinatal period.