Lysine E-amino group reacts with citraconic anhydride forming a derivative, which is stable on terms for trypsin cleavage. This modification changes the spectrum of peptides formed by the trypsin action; as the number of trypsin-sensitive sites is reduced, the peptides with higher molecular mass can survive in the digest. The various studies of proteins by MALDI-TOF mass spectrometry are often complicated by the low sequence coverage of the peptide chain. This paper demonstrates that the modification of proteins by citraconylation before trypsin cleavage represents a simple experimental technique, which allows a significant increase of sequence coverage in MALDI-TOF mass spectrometry. This improvement is caused both by change of trypsin fragmentation pattern and by disturbance of the protein's native tertiary structure. (C) 2003 Elsevier Science (USA). All rights reserved.