Abstract: The present invention relates to a method for cross-linking peptides using an activated furan-moiety. In particular, the present invention provides a method for cross-linking peptides comprising the steps of: a) providing a composition comprising furan-peptides, said furan-peptides comprising at least one amino acid comprising a furan-moiety; b) contacting said composition comprising furan-peptides with second peptides, thereby obtaining a mixture comprising furan-peptides and second peptides; c) adding an activation signal to said mixture of step b), thereby activating said furan-peptides to activated furan-peptides, and d) reacting said activated furan-peptides with said second peptides, thereby cross-linking said activated furan-peptides with said second peptides.