During the last several years, studies in humans and experimental animals have identified several compounds that induce fetal hemoglobin in the adult. These include: cell-cycle-specific drugs, other cytotoxic drugs, butyric acid analogs, and erythropoietin. Several of these compounds induce fetal hemoglobin indirectly by triggering kinetics of rapid erythroid regeneration. High doses of erythropoietin increase the frequency of erythroid progenitors programmed to hemoglobin F. This results in transient increases of hemoglobin F-containing cells (F cells) in the peripheral blood. Erythropoietin and hydroxyurea increase F cells in a cooperative fashion. Although high doses of erythropoietin can induce F cell production in humans, the practical relevance of such observations is unclear.