Treatment of peptides with excess HgO in the presence of alkaline cyanide leads to cleavage of the peptides at glycine residues. The reaction appears to involve both C- and N-mercuration with subsequent release of 2 mol mercury per mol of glycine. An intermediate glyoxylic acid residue in Schiff base linkage is postulated. Treatment of the heptapeptide Phe-Ala-Lys-Gly-Leu-Asp-Val with alkaline HgO and KCN for 6 h at 25 degrees resulted in greater than 90% cleavage, and the resultant reaction products were separated by reverse phase chromatography and identified by amino acid analysis. N-terminal products were approximately equimolar Phe-Ala-Lys, Phe-Ala-Lys-Gly, and Phe-Ala-Lys-amide. C-terminal products were predominantly Leu-Asp-Val (63%), plus Gly-Leu-Asp-Val (9%), and oxalyl-Leu-Asp-Val (8%). This method may be useful for cleavage of peptides or proteins containing glycine residues.