Granulocyte colony-stimulating factor (G-CSF) is the principal growth factor regulating the production of neutrophils, yet its role in lineage commitment and terminal differentiation of hematopoietic progenitor cells is controversial. In this study, we describe a system to study the role of G-CSF receptor (G-CSFR) signals in granulocytic differentiation using retroviral transduction of G-CSFR–deficient, primary hematopoietic progenitor cells. We show that ectopic expression of wild-type G-CSFR in hematopoietic progenitor cells supports G-CSF–dependent differentiation of these cells into mature granulocytes, macrophages, megakaryocytes, and erythroid cells. Furthermore, we show that two mutant G-CSFR proteins, a truncation mutant that deletes the carboxy-terminal 96 amino acids and a chimeric receptor containing the extracellular and transmembrane domains of the G-CSFR fused to the cytoplasmic domain of the erythropoietin receptor, are able to support the production of morphologically mature, chloroacetate esterase-positive, Gr-1/Mac-1–positive neutrophils in response to G-CSF. These results demonstrate that ectopic expression of the G-CSFR in hematopoietic progenitor cells allows for multilineage differentiation and suggest that unique signals generated by the cytoplasmic domain of the G-CSFR are not required for G-CSF–dependent granulocytic differentiation.