The human leukemia K562 cell line can be induced by 20 micro M hemin to reversibly accumulate embryonic and fetal hemoglobins without any change in the rate of cell division. When we reduced the rate of cell division by glutamine starvation or addition of hydroxyurea, the cells increased by tenfold the basal hemoglobin level of 0.3–0.5 pg Hb/cell. The combined effects of hemin and inhibitors of cell division permitted K562 cells to attain levels of hemoglobin (26–34 pg Hb/cell) close to that found in normal red cells. This superinduction was reversible and cells could be recycled indefinitely. Furthermore, electrofocusing experiments show that the three primary hemoglobin species produced by these cells (Hb Gower 1, Hb Portland, and fetal Hb), were induced, or reinduced, synchronously by inhibitors of cell division but asynchronously by hemin. Differing effects of hemin and inhibitors of cell division were observed in the absence of irreversible differentiation and suggest different molecular mechanisms controlling globin gene expression.