The structural characterisation of a synthetic peptide reproducing the sequence 1-30 of Par j 1.0101, a major allergenic protein present in the pollen of Parietaria judaica, by combined use of chemical and enzymatic cleavage, reversed-phase high-performance liquid chromatography and electrospray ionisation mass spectrometry (ESI-MS), is described. Direct ESI-MS of the synthetic peptide after reaction with methyl iodide showed that the product is a mixture of two peptides: one form in which two out of the four cysteine residues present in the sequence are oxidised and a minor amount of another form in which all the cysteines are fully reduced. It was ascertained, using the combined procedure indicated above and without prior separation of the two species, that the disulphide bond in the partially oxidised form is located between cysteines 29 and 30. These results show the usefulness of this approach for characterising synthetic peptides containing multiple cysteine residues in the sequence.