Abstract: The folding/oxidation of a reduced peptide or partially reduced peptide to form a disulphide bridged peptide is effected by dissolving it in an oxidizing organic solvent, alone or in admixture with water, adding an aqueous alkaline buffer to the solution, and recovering the resultant disulphide bridged peptide. The preferred oxidizing organic solvent is dimethylsulphoxide, which is desirably used as a 10 to 50% aqueous solution. The addition of the aqueous alkaline buffer, which is preferably a 0.2 M Tris-HCI buffer, is preferably added during a period of from 5 to 90 minutes after dissolution of the reduced peptide in the oxidizing organic solvent. The method allows reduced peptides which are insoluble in alkaline conditions to be oxidized and allows reduced peptides which may form stable but inactive oxidized species if treated with dimethylsulphoxide alone to be fully oxidized.