Peracid oxidation of wool or bovine plasma albumin greatly increases the apparent thiol contents of the proteins as estimated by reaction with organomercuric halides. This apparent anomaly results principally from reaction of the mercurials with disulphide monoxide (thiolsulphinate) groups present (as S-monoxycystyl residues) in the protein. SS-Dioxycystyl residues may also react under some conditions. The mercurial reagents similarly combine with cystine (�)-S-monoxide.