Recombinant human erythropoietin (rHuEPO) is used abundantly in the clinic to stimulate red blood cell growth in anaemic patients. The efficacy of the drug depends strongly on the extent of sialylation of its carbohydrate moiety. Prompted by conflicting literature reports on the issue, we reinvestigated the structures of the intact sialylated carbohydrate chains of rHuEPO expressed in Chinese hamster ovary (CHO) cells. The asparagine-linked oligosaccharides were released from rHuEPO with N-glycanase and fractionated by anion-exchange chromatography. The O-linked oligosaccharides were released under alkaline borohydride conditions. The primary structures of the major sialylated N- and O-type oligosaccharides were identified by 500-MHz 1H-NMR spectroscopy, supported by data from composition analysis, methylation analysis, low- and high-pH anion-exchange chromatography, and fast atom bombardment-mass spectrometry. The most abundant N-linked oligosaccharides in CHO cell-derived rHuEPO were found to be di-antennary, 2,4-branched tri-antennary, 2,6-branched tri-antennary and tetra-antennary chains (in the ratio of 7:6:5:82), with the latter containing between zero and three repeating N-acetyllactosamine units, in well-defined branches. The major (> 95%) di-, tri- and tetra-antennary structures are fully sialylated, i.e. they have two, three and four sialic acid residues, respectively, linked exclusively alpha(2-->3) to galactose residues. The majority (> 95%) of N-linked structures contain alpha(1-->6)-linked fucose at the proximal GlcNAc residue. The O-type mono- and disialyl oligosaccharides were characterized as a linear tri- and a branched tetra-saccharide, respectively.