Marine mussels affix themselves to surfaces by use of a highly cross-linked, protein-based adhesive. Metal levels (e.g., Fe, Zn, Cu, Mn) of the cured glue are significantly concentrated relative to surrounding waters. Specific details on the reagents used by mussels to induce protein cross-linking are not known at this time. To provide insight on the cross-linking agents and reactions taking place while curing mussel glues, we performed a study in which various compounds were tested for the ability to bring about protein curing. A precursor to adhesion, with proteins containing the unusual amino acid 3,4-dihydroxyphenylalanine, was extracted from mussel feet. Potential cross-linking agents were mixed with this gelatinous pellet. The compressibility and shear properties of the resulting material were investigated by use of a penetration test. The reagents examined included simple metal ions (e.g., Na+, Zn2+), oxidizing transition metals (e.g., Fe3+, Cr2O7(2-)), nonmetallic oxidants (e.g., H2O2,IO4-), and oxidizing enzymes (e.g., tyrosinase). We found that protein curing was brought about by simple oxidants and transition metal ions. The results show that optimal curing occurs when the reagent is an oxidizing metal ion (e.g., MnO4-, Fe3+). We conclude that marine mussels are likely to employ Mn3+ and Fe3+ for protein cross-linking and adhesive synthesis.