Monitoring occupational exposure to styrene was achieved through quantification of adducts of styrene 7,8-oxide to N-terminal valine in hemoglobin (Hb) on the basis of the enrichment of adducted globin chains by ion-exchange chromatography and gas chromatographic-mass spectrometric analysis by the use of the N-alkyl Edman method. Application to blood samples from reinforced plastics workers exposed to styrene and from referents showed Hb adduct levels correlating with the blood styrene glycol and urinary mandelic acid concentrations. The blood styrene glycol and styrene 7,8-oxide levels of the exposed workers averaged 2.5 mumol.l-1 (17 subjects) and 0.09 mumol.l-1 (7 subjects), respectively. The blood styrene glycol and urinary mandelic acid content (mean 9.5 mmol.l-1, 17 subjects) suggested a styrene concentration of about 300 mg.m-3 (75 ppm) in the workplace air. The Hb adduct levels were low (mean 28 pmol.g-1), indicating rapid detoxification of styrene 7,8-oxide in humans.