Typically, alpha-2,3-sialyllactose is used as the sialic acid donor in trans-sialidase catalyzed reaction. However, due to limitations such as reversibility and cost, alternative sialic acid donors are needed. Here we show that fetuin, a glycoprotein containing abundant sialic acids at the ends of its oligosaccharides, can be used as a sialic acid donor in trans-sialidase catalyzed reaction. Among 166 nmol of total sialic acid in milligrams fetuin, 125 nmol of sialic acid was consumed for the trans-sialidase reaction. The trans-sialidase reaction using fetuin was reversible. The sialyl transfer rate of fetuin to Galbeta(l,4)GlcNAc was similar to that of a-2,3-sialyllactose and approximately 30-40 times greater than that of 4-methylumbelliferryl-alpha-sialic acid. trans-Sialidase reaction was performed using 200 mg of fetuin and 34 mg of lactose as a donor and an acceptor, respectively, and 8 mg of product, i.e. alpha-2,3-sialyllactose, was purified by gel filtration column. To simplify the purification step, trans-sialidase reaction was carried out by submerging and stirring a dialysis bag containing fetuin and trans-sialidase into a lactose solution.