Reactivation of fetal hemoglobin gene expressions was studied in new-world monkeys (marmosets). After a series of intraperitoneal injections of D-thyroxine, fetal hemoglobin production was increased in three experimental animals. Control animals were unaffected by the injection of the solvent. Because the percentage of cells containing fetal hemoglobin exceeded the total percentage of fetal hemoglobin, it was concluded that thyroid hormone influenced the synthesis of fetal hemoglobin rather than the development of a population of fetal cells.