The p-methoxybenzenesulfonyl (MBS) group was used to protect the ω-guanidino function in arginine and was found to be more easily and completely removed by treatment with methanesulfonic acid and borontristrifluoroacetate (BTFA) than the p-toluene-sulfonyl group. In order to evaluate the usefulness of this new protecting group in peptide synthesis, bradykinin and tuftsin were prepared by using the MBS group.