3-Nitro-2-pyridinesulfenyl group (Npys) is a classical protective group developed in 1980,¹ particularly for protecting a thiol (SH) group with a unique characteristic; “-Cys(Npys)-“ readily reacts with an unprotected SH group to form a disulfide bond.^2,3 Based on this chemistry, we recently developed SH-selective solid-supported biotinylation and oligoarginylation reagents as tools of Chemical Biology.^4,5 A compound with an SH group can be selectively labeled and the resulting labeled compound can be released from the resin and easily recovered in a high purity by filtration. Next, using a synthesized “Npys chloride resin”, we developed a “solid-phase disulfide ligation” method to prepare a disulfide peptide from two kinds of peptide fragments with cysteine residues.⁶ Application of this strategy realizes a “disulfide-led synthetic methodology” for disulfide-containing cyclic peptides. Oxytocin was efficiently synthesized as a fundamental model for more complex cyclic peptides. We would like to discuss further about the application of this method to the synthesis of Endothelin with two disulfide bonds and a noncovalent–type antibody–drug conjugate (NCADC) mediated by Fc-binding peptide in Medicinal Chemistry. Finally, as a more recent study, we also discuss a new solid- or no-solid-supported Npys reagent (Npys-OR) that enhances intra-molecular disulfide bond formation between two cysteine residues for the effective synthesis of cyclic-disulfide peptides. In use of this mild oxidizing agent, we successfully synthesized hANP and α-conotoxin without oligomer formation under the mildly acidic conditions with a relatively higher peptide concentration.