In order to investigate if hemoglobin might serve as a biomarker of exposure for 1,2- dichloroethane (DCE) encountered in the workplace, human hemoglobin was alkylated at physiologic pHby the episulfonium ion of S-(2-chloroethyl)glutathione (CEC). In vitro alkylation resulted in three alkylation products on the α chain and at least two alkylation products on the βl chain as determined directly by matrix-assisted laser desorption-ioniza tion mass spectrometry. To ascertain if the site of alkylation was the reactive sulfhydryl present at cysteine-93 on the β chain of hemoglobin (β-93Cys), a spectrophotometric assay using 4,4'-dithiodipyridine was used to measure the free sulfhydryl groups before and after treatment of hemoglobin with various amounts of CEC. Results indicate that the episulfonium ion did not react substantially at β-93Cys, as there was no measurable decrease in the sulfhydryl to hemoglobin ratio, even with a large excess of CEC. In contrast, iodoacetamide did react with the sulfhydryl groups and gave a dose-dependent decrease in the sulfhydryl to hemoglobin ratio as measured by this assay. CEC-treated hemoglobin was digested with Staphylococcus aureus endoproteinase Glu-C and the digest was analyzed by fast atom bombardment mass spectrometry. Only one peak in the FAB mass spectrum could correspond to a peptide modified by the episulfonium ion of CEC. These results indicate that although the episulfonium ion of CEC does alkylate human hemoglobin, β-93Cysis not the major alkylation target.