We succeeded in the expression, purification, and refolding of the immunoglobulin-like (Ig) domain of human granulocyte-colony-stimulating factor (G;-CSF) receptor with amino-terminal His-tag in Escherichia coli. The refolded Ig domain bound to a G;-CSF affinity column and could be eluted with free G-CSF as a receptor-ligand complex, demonstrating that the Ig domain has the information necessary for binding its ligand, G-CSF. The eluted His-Ig/G-CSF complex could be separated from excess G-CSF by Ni-NTA column chromatography. The yield of this active recombinant His-Ig protein is about 0.72 mg per liter of culture. Its small size and the ease of production make this receptor fragment a useful reagent for the structural analysis of its complex with G-CSF.