A new method is described for the inter‐ and intramolecular cross‐linking of the tryptophan residues in polypeptides and proteins. The reaction employed involves treatment of the tryptophan‐containing derivative with 2,4‐dinitro‐1,5‐phenyldisulfenyl chloride (DNPDS‐Cl)* in acidic media. This bifunctional sulfenyl halide reacts selectively with two molecules of tryptophan at the position of indole nucleus, leading to a cross‐linked compound. The only other amino acid affected is cysteine, which similarly can be cross‐linked through formation of mixed disulfide bonds. The reaction with the thiol function of cysteine can be reversed by reduction with mercaptans, allowing selective cross‐linking of tryptophan residues.In order to test the effectiveness of the method, several tryptophan derivatives were cross‐linked by reaction with DNPDS‐Cl in acetic or formic acid, including short sequences of the pancreatic hormone glucagon and gastrin.The reaction was also applied to glucagon, in which case the single tryptophan residue was reacted with the reagent and the dimer isolat