Granulocyte colony-stimulating factor (G-CSF) is a glycoprotein which stimulates predominantly neutrophilic granulocyte colony formation in mammals. Natural human G-CSF (hG-CSF) and recombinant hG-CSF produced in Chinese hamster ovary (CHO) cells transfected with the cDNA clone for hG-CSF have been purified to apparent homogeneity for structural and biological comparison. The amino acid sequence of recombinant hG-CSF, composed of 174 amino acid residues, was identical with that of natural hG-CSF and also with the sequence predicted from the cDNA. Both forms of hG-CSF have a free Cys-17 and two intramolecular disulfide linkages, between Cys-36 and Cys-42, and between Cys-64 and Cys-74. The O-glycosylation occurred at Thr-133 in both hG-CSFs. Similar CD spectra were obtained for both hG-CSFs. Additionally, both forms showed almost the same biological activities determined by in vitro colony-forming assay and in vivo assay. It is thus concluded that the recombinant hG-CSF is indistinguishable from its natural counterpart and that the former is valuable for more detailed characterization and clinical use.