Chloramine-T (p-toluene sulfonochloramine) is an effective method of labeling a variety of proteins and peptides containing Tyrosine or Histidine. Chloramine-T breaks down slowly in a slightly alkaline (pH 7.5) aqueous solution, forming hypochlorous acid. Hypochlorite acts as an effective oxidizing agent where [125I]-Na forms cationic iodine (I+) which is readily taken up by available tyrosine residues in the protein. The iodine atoms substitute ortho to the hydroxyl group on the phenolic ring of tyrosine. Above pH 8 – 8.5, the substitution of iodine into the imidazole ring of histidine is favoured which can be useful if the tyrosine residues are present within the biologically active portion of the protein. Chloramine-T produces proteins or peptides labeled with carrier-free radioiodine with high specific activity but can be harsh.