It is important to study the hydrogen-bonding character in hydrophilic polymers in detail to create new hydrophilic synthetic polymers controlled by hydration. In this paper, the role of hydroxyl side chains in sericin, a gluelike hydrophilic protein of Bombyx mon cocoon, in forming stable β-sheet aggregates is studied on the basis of 13C solid-state NMR analyses. To clarify the molecular mechanism to stabilize the solid structure of sericin, 13C CP/MAS and DD/MAS NMR measurements of native sericin and a model peptide of its crystal domain were performed at hydrated state. There are hydroxyl side chains of Ser and Thr residues in β-sheet structure which remain rigid after hydration. These hydroxyl side chains were assumed to clump adjacent β-sheets by hydrogen bonding at their OH groups. We inferred that Ser and Thr linkage sequences in the crystal domain are responsible for forming β-sheet aggregates and generating the structural stability of sericin. © 2007 American Chemical Society.