The disulfide bond in S-3-nitro-2-pyridinesulfenyl (S-Npys) compounds is stable towards the acid treatment used in solid-phase peptide synthesis, yet the liability of S-Npys-peptides towards nucleophiles enables the conjugation to proteins to proceed under mild conditions. Thus Boc-Cys(Npys)-OH was coupled as N-terminal residue to a resin-linked peptide chain. After deprotection and cleavage from the resin the Npys-cysteinylpeptide was attached to a properly functionalized protein by reaction with a mercapto group. The amount of peptide conjugated to the protein was determined by measuring the amount of 3-nitro-2-thiopyridone liberated. The cysteinylpeptide which was detached from the protein by reduction of the disulfide bond was shown to be identical with the product obtained by reduction of the Npys-cysteinylpeptide.