CD34+ cells express the basic helix-loop-helix transcription factor SCL, which is essential for blood cell formation in vivo. In addition, their survival is critically dependent on hemopoietic growth factors. We therefore compared the effects of Steel factor (SF) and GM-CSF on the survival, proliferation, and differentiation of primary human CD34+ cells, as well as the role of SCL during these processes. GM-CSF suppresses apoptosis in CD34+ cells, which proliferate and differentiate into mature granulocytic and monocytic cells (CD34-CD13+) and loose SCL expression. In contrast, SF suppresses apoptosis without a significant increase in cell numbers, and the cells remain CD34+ and SCL+ with a blast-like morphology. Examination of apoptosis by the terminal deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling (TUNEL) reaction and of the cell cycle status indicated that SF is both a survival factor and a mitogenic factor for CD34+ cells. There was, however, constant cell death in a fraction of the population, which could be rescued by GM-CSF. Co-addition of SF and GM-CSF prevents the downregulation of SCL observed in the presence of GM-CSF by itself, allows for prolonged survival and expansion of CD34+ cells in culture, inhibits monocytic differentiation and impairs granulocytic differentiation. Finally, exposure to an antisense SCL but not a control oligonucleotide decreases SCL protein levels and prevents the suppression of apoptosis by SF without affecting GM-CSF-dependent cell survival. These observations suggest that the hemopoietic transcription factor SCL regulates the survival of CD34+ cells in response to SF.