A novel chemoenzymatic method has been developed for the synthesis of size-defined polysaccharides with sialic acid-containing repeating units by sialyltransferase-catalyzed block transfer of oligosaccharides. The synthesized polysaccharides are mimics of surface polysaccharides produced by some pathogenic bacteria. Bacterial sialyltransferases, once again, have been confirmed to have flexible substrate specificity. They are extremely useful catalysts for efficient synthesis of diverse and complex sialic acid-containing carbohydrates and glycoconjugates.