Argentinated peptide ions are formed in abundance under matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) conditions in the presence of Ag+ ions. These argentinated peptide ions are fragmented facilely under MALDI-MS/MS conditions to yield [b(n) + OH + Ag]+, [b(n) - H + Ag]+ and [a(n) - H + Ag]+ ions that are indicative of the C-terminal sequence. These observations parallel those made earlier under electrospray MS conditions (Chu, I. K; Guo, X.; Lau, T.-C.; Siu, K W. M. Anal. Chem. 1999, 71, 2364-2372). A mixed protonated and argentinated tryptic peptide map was generated from 37 fmol of bovine serum albumin (BSA) using MALDI-MS. MALDI-MS/MS data from four argentinated peptides at a protein amount of 350 fmol unambiguously identified the protein as BSA. Sequence-tag analysis of two argentinated tryptic peptides was used to identify unambiguously myocyte enhancer factor 2A, which had been recombinantly expressed in a bacterial cell line.