Thiol and disulphide groups in proteins react with neutral and alkaline cupric sulphite solutions and are thereby converted into thiolsulphate (-SSO3H) groups. Solutions of cuprammonium sulphite at pH 9-10.6 can dissolve large amounts of keratins ; under other conditions the disulphide bonds can be broken without the proteins entering solution. The reactions are highly specific, proceed to completion, and large excesses of reagent are not required. The " S-sulphokerateines " so obtained constitute a new class of water-soluble protein derivative. The thiolsulphate group can be readily labelled with radioactive sulphur and may be converted into a thiocyanate (" S-cyano-kerateine ") by reaction with cyanide, and into a mixed disulphide by reaction with a thiol. S-Cyanokerateines, containing the ?CH2SCN side chain, react with cysteine to give thiocyanate ion and a protein containing lanthionine residues, presumably by nucleophilic displacement of SCN- from the CH2 group.