N-linked glycoprotein glycans were modified by combined use of glycosidases and glycosyltransferases. Human milk 81,4 galactosyltransferase andrat liver 02,6 sialyltransferase were immobilized by covalent or affinity binding methods. These immobilized glycosyltransferases were used in a slurry reactor for continuous glycosylation of endoglycosidase H-treated invertase or r-tPA. Theresulting sialylated glycan chains mimicking the structureof outer complex N-glycansconferred extended plasma half-life time compared to galactosylated glycoprotein.