Abstract: The invention relates to a method of cross-linking poly(amino acid) compounds (e.g. proteins, polypeptides, protein polymers, etc.) to each other or attaching such compounds to polyamino or polycarboxyl compounds. The method involves lyophilizing a solution of at least one poly(amino acid) compound, or at least one poly(amino acid) compound and a polyamino or polycarboxyl compound, maintaining the lyophilized solid under vacuum, heating the lyophilized solid under vacuum to an elevated temperature effective to cause cross-linking without denaturing of the poly(amino acid) compounds, and cooling the product and releasing the vacuum. The method produces cross-linking without the use of activating compounds or cross-linking molecules. The invention also relates to novel cross-linked products.