The mesitylenesulfonyl (Mts) group was introduced at the Nin of tryptophan by Illi's method. This group is stable under various conditions required for practical peptide synthesis, such as treatments with dil. alkali, hydrazine hydrate, TFA and even 4 N HCl-dioxane or 25% HBr-AcOH. HF is not able to remove this protecting group, but it can be smoothly cleaved by 1 M trifluoromethanesulfonic acid/TFA or methanesulfonic acid. The usefulness of this new tryptophan derivative, Trp (Mts), for practical peptide synthesis was demonstrated by the synthesis of cholecystokinin-heptapeptide as an example.