A method is reported for measuring cysteinyl adducts of the benzene metabolite benzene oxide (BO) with hemoglobin (Hb) in blood from humans or rodents exposed to benzene. After reacting the purified, dried protein with trifluoroacetic anhydride and methanesulfonic acid, the resulting phenyltrifluorothioacetate is extracted, washed, and detected by GC-MS in the negative-ion chemical ionization mode. The analysis of Hb adducts of BO from rats dosed with 50-400 mg/kg [13C6]benzene via this method resulted in values which were generally consistent, though slightly lower, than those obtained using an established method. However, while 3 weeks were required to process the samples by the former method, only 2 days were needed with the new procedure. This new method should, therefore, prove to be reliable and convenient for the rapid quantitation of cysteine-bound Hb adducts of BO.