In this paper we describe the use of tandem mass spectrometry to identify modified sites in human hemoglobin after in vitro exposure to bis(2- chloroethyl) sulfide (sulfur mustard). Globin isolated from human whole blood which had been exposed to sulfur mustard was degraded with trypsin, and the digests were analyzed by micro LC/MS. Alkylated tryptic fragments (α-T1, α- T4, α-T6, α-T9, β-T1, β-T9, β-T10, β-T11, and β-T10-S-S-β-T12) could be tentatively assigned upon comparison with a digest from nonexposed globin. Subsequent tandem mass spectrometry of these peptides allowed unambiguous assignment of 5 specific modified residues: α-Val-1, α-His-20, β-Val-1, β-His-77, and β-His-97. The results demonstrate the usefulness of microbore LC in combination with tandem mass spectrometry for the structural determination of chemically modified peptides and proteins.