Abstract Erythroid blasts are progenitors that can differentiate to mature red blood cells (RBC). Identification of these growth factors and understanding their downstream pathways can help us to optimize RBC production ex vivo. We derived a stromal cell line (PL16) from mouse embryonic fetal livers that promotes proliferation of mouse and human erythroid blasts in vitro. Among specifically highly expressed secreted proteins in PL16, we identified a new growth factor (FA) that is capable of stimulating human erythroid blast expansion in vitro significantly. Furthermore, erythroid blasts from our in vitro expansion system can differentiate into functional mature red blood cells. Disclosures: No relevant conflicts of interest to declare.