A series of truncated Herpes simplex virion peptides studied by fast atom bombardment mass spectrometry under high and low energy collision induced dissociation conditions showed preferential fragmentation of the aspartyl-proline amide bond, compared to other peptide bonds. Electrospray ionization investigation proved that this favoured fragmentation can not be attributed to only the known proline effect, as a change from Asp to Asn in the peptide yielded an Asn-Pro bond which was found to be stable under the same ionization conditions. This mass spectrometric behaviour is in good agreement with the observation that DP bonds are sensitive to acidic conditions.