Alkylation of DNA by xenobiotic agents, or their electrophilic metabolites, is believed to be the major initiating process that may result ultimately in carcinogenesis. The study of hemoglobin alkylated in vivo by chemical carcinogens has previously been proposed as an indicator for DNA alkylation. Xenobiotically modified proteins, however, are not readily amenable to conventional methods for amino acid sequencing. Tandem mass spectrometry allows unambiguous structural elucidation of chemically modified proteins. Styrene is a widely used chemical in the plastics industry and its major metabolite, styrene 7,8-oxide, is both mutagenic and carcinogenic in rodents. Human hemoglobin was modified in vitro with styrene 7,8-oxide and digested with trypsin. Tryptic peptides from unmodified hemoglobin were isolated by high performance liquid chromatography, and their molecular weights were determined by liquid secondary ion mass spectrometry. This allowed confirmation of the known sequence of the protein and provided a reference for the identification of modified peptides. High performance tandem mass spectrometry of modified peptides allowed unambiguous assignment of specific residues modified. The externally accessible histidines were found to be the dominant sites for alkylation at high modification levels of the protein.