N-Acetylneuraminic acid (NeuAc), the representative of the family of sialic acids, is an important molecule in biological recognition systems. Currently, NeuAc-based novel pharmaceutical agents and diagnostic reagents for influenza viruses are highly required in medical fields, and larger amounts of NeuAc are in demand worldwide. NeuAc had been prepared either from colominic acid (a homopolymer of NeuAc) produced by fermentation or from natural sources such as edible bird's nests, milk or eggs. However, the drawbacks of such conventional methods make them unsuitable for large-scale production of NeuAc. Recently, the N-acylglucosamine 2-epimerase (AGE) gene from porcine kidney was cloned in E. coli, and a strain with a high AGE expression level was constructed for practical applications, which enabled the complete enzymatic synthesis of NeuAc with a high conversion rate from the substrates, namely, N-acetylglucosamine and pyruvate. In addition, NeuAc of highest purity could be produced economically via its direct crystallization from the reaction mixture without any column purification processes. Such a simple procedure promises to be applicable to the mass production of sialic acid at the lowest cost.