Circulating erythropoietic precursors in normal men and patients with hemoglobinopathies were characterized in culture. Blood mononuclear cells harvested with a modification of the Ficoll-Isopaque technique were cultured in methylcellulose for 14 days. The majority of erythropoietic colonies consisted of several subcolonies assuming the morphology of erythropoietic “bursts” described in murine marrow cultures. Time course studied of colony formation from marrow and blood nucleated cells confirmed that the circulating erythropoietic precursors represented only early stages of development. Peak sedimentation velocity of the circulating precursors analyzed using a Staput apparatus averaged 5.31 mm/hr and corresponded with that of the early erythropoietic precursors in human marrow. One ml of blood yielded an average of 153 colonies in normal men and 785 colonies in patients with hemoglobinopathies. No correlation was observed between colony formation and reticulocyte indices of individual patients. Examination of the proliferative state of the erythropoietic precursors using high specific activity tritium-labeled thymidine revealed that almost none of the cells in normal men or patients with hemoglobinopathies were in the DNA synthetic phase.