In the present study, the purification and sequence analysis of a peptide extracted from human plasma is described. After three chromatographic steps in one of the plasma fractions, a molecular mass of 920 Da was determined with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis. Further analysis with post-source-decay (PSD)-MALDI-MS revealed an amino acid sequence arg-pro(pro-gly+16)-phe-ser-pro-phe. With the data given from the spectrum, it was impossible to clarify the partial sequence (pro-gly+16). A comparison with a data base suggested that the sequence is very similar to des-arg9bradykinin (arg-pro-pro-gly-phe-ser-pro-phe). But many fragment masses, including the mass of the parent ion of the unknown peptide, are 16 Da larger than the corresponding fragment masses of des-arg9-bradykinin. The interpretation of the fragment pattern does not allow determination of the position of the amino acid that is 16 Da larger. With automated Edman sequence analysis of the fraction of interest, three more major component peptides in this fraction were observed, which were not detected with MALDI-MS. Nevertheless, with the help of the Edman sequence data it was possible to identify the unknown amino acid in the PSD-MALDI-MS sequence as hydroxyproline at the third position. In conclusion, the existence of des-arg9[hyp3]-bradykinin (arg-pro-hyp-gly-phe-ser-pro-phe) in human plasma is described. Furthermore, the study demonstrates that PSD-MALDI-MS and automated Edman sequence analysis complement each other if single unknown peptides from mixtures are to be sequenced.