Abstract: Carbonyl-bis (L-methionine p-nitrophenyl ester) and its analogues are useful reagents for the intermolecular cross-linking of two chain peptide molecules such as occur in the hormone insulin. The cross-linking occurs across amino groups at preferred positions in the two peptide chains to fix the relative spacial position of the peptide chains and permit the oxidative formation of necessary disulfide bridges between the chains in high yields. The reagent is then removed from the linked chains to yield the insulin molecule. A method for preparation of the reagent is disclosed. Further, a procedure is disclosed by which the cross-linking reagent is used to specifically block certain amino groups on insulin to yield a cross-linked insulin which in turn is used to prepare isotopically labeled insulin and insulin analogues.