Our aim is to understand mechanisms involved in globin gene regulation, particularly developmental switching. To study regulatory sequences of the globin gene promoter and flanking region, we constructed a series of composite promoters, each containing a fragment from the fetal gamma-globin upstream flanking region joined to a beta-globin promoter. Their function was assessed in stably transformed human K562 cells, an erythroid cell line that expresses the gamma-globin gene but not the beta. A gamma fragment spanning positions -259 and -137 activated the nonfunctional beta promoter in these cells. Further analysis of this 120 bp sequence may identify elements controlling the switching of the fetal gamma-globin gene.