Production of glycoproteins using manganese

A culture medium comprising mammalian host cells and a non-toxic amount of manganese effective to increase the sialylation of a glycoprotein composition, wherein said host cells have been transfected with DNA encoding erythropoiesis-stimulating molecules comprising erythropoietin (SEQ ID NO:

• 3) or darbepoetin (SEQ ID NO:
• 2), or analogs thereof with 75% homology to SEQ ID NO:
• 3 or SEQ ID NO:
• 2, respectively, and still retaining erythropoietic activity, wherein the concentration of manganese is about 40 μM, and wherein less than about 5% of the secreted erythropoiesis-stimulating molecules are lower sialylated.