A cytokine-screening assay of cultured peripheral blood cells obtained using immune rosetting and separation of progenitors was developed to identify determinants of fetal hemoglobin (HbF) modulation during adult erythropoiesis. Among the 12 erythroid growth-promoting cytokines tested, stem cell factor (SCF) at a concentration of 50 ng/mL resulted in the most significant increase in cell proliferation and HbF content. The average HbF/hemoglobin A (HbA) ratio was 30.9% +/- 18.7% in cultures containing SCF compared with 4.1% +/- 2.2% in those grown with erythropoietin (EPO) alone (P = 8.5E-8). To further investigate the hemoglobin-modulating effects of SCF, we examined the surface expression pattern of the SCF receptor, CD117, among maturing erythroblasts. CD117 expression increased during the first week of culture and peaked on culture days 7 to 9. After culture day 9, the level of CD117 declined to lower levels. The rise in CD117 expression to high levels mirrored that of the transferrin receptor (CD71), and the subsequent reduction in CD117 was inversely related to increases in expression of glycophorin A. SCF-related increases in the HbF/HbA ratio correlated with the expression pattern of CD117. SCF added during days 7 to 14 resulted in a more pancellular distribution of HbF on day 14 compared with the heterocellular distribution present in cultures supplemented with SCF on days 0 to 7. A significant SCF-mediated increase in HbF was also measured using progenitors derived from cord blood. These results suggest that the HbF response to SCF is greatest at the late progenitor stage as a function of surface CD117 expression.