A new method for the effective reduction of methionine sulphoxide in protected peptides with sulphur trioxide and thiol has been developed. In practice, a combination of dimethylformamidesulphur trioxide complex and ethane-1,2-dithiol was found effective, the presence of pyridine in the reaction medium being found necessary for the reduction to proceed. Methionine sulphoxide in two types of protected methionine-enkephalins was efficiently reduced to methionine by this reduction system at 20 °C in an hour. The (p-methylsulphinyl)benzyl protecting group was also reduced to (p-methylthio)benzyl simultaneously with the reduction of methionine sulphoxide. Subsequent deprotection afforded methionine-enkephalin of high purity in each case.