The purpose of this study was to compare different biological methods in current use to assess benzene exposure. The methods involved in the study were: benzene in blood, urine and exhaled air, and the urinary metabolites t,t-muconic acid (MA) and S-phenylmercapturic acid (S-PMA). Blood, urine and exhaled air samples were collected from workers in a benzene plant (pure benzene exposure) and cokery (mixed exposure, e.g. polycyclic aromatic hydrocarbons--PAHs) in an Estonian shale oil petrochemical plant. The benzene in these samples was analysed with a head-space gas chromatograph, and the metabolites MA and S-PMA with a liquid chromatograph using methods developed from published procedures. Some of the values measured in the Estonian shale oil area were high in comparison with those published during the last few years, whereas the values measured in the control group did not show any exposure to benzene except in the smokers group. The highest median exposure was in the benzene factory, 0.9 cm3/m3 TWA (2.9 mg/m3) and the highest individual value was 15 cm3/m3 TWA (49 mg/m3). All biological measurements in this study gave the same assessment about exposure to benzene and correlated highly significantly with each other and with the air measurements (r = 0.8 or more). In the benzene factory the correlation was good even when calculated from samples with air concentration < 1 cm3/m3 (3.2 mg/m3) in the case of blood benzene and urinary MA. However, for S-PMA it was weak (r = 0.4) and for benzene in urine and exhaled air it did not exist any more. In the cokery, with mixed exposure, the correlation at low levels was weaker even for blood benzene and urinary MA (r = 0.6). According to the results in the benzene factory the exposure to pure benzene at the level 1 cm3/m3 (3.25 mg/m3) TWA gave: the blood benzene value about 110 nmol/l (8.6 micrograms/l), MA 23 mumol/l (3.3 micrograms/l) or 2.0 mg/g creatinine, S-PMA 58 micrograms/g creatinine or 0.4 mumol/l (95.7 micrograms/l), benzene in urine 499 nmol/l (39 micrograms/l), and benzene in the exhaled air 2.8 nmol/l (0.2 microgram/l). In general, the measurement of benzene in blood and in exhaled air, as well as benzene and its metabolites MA and S-PMA in urine, all gave similar results. However, at low exposure level (< 1 cm3/m3) the most reliable analyses were MA in urine and benzene in blood.