S-phenylmercapturic acid (S-PMA) was measured in urine from 145 subjects exposed to low benzene concentrations in the air (C(I), benzene). The 8-h, time-weighted exposure intensity of individual workers was monitored by means of charcoal tubes and subsequent gas-chromatographic analysis after desorption with CS2. S-PMA excretion level in urine was determined by high-performance liquid chromatography with fluorescence detection. The following linear correlation was found between S-PMA concentrations in urine and benzene concentrations in the breathing zone: log(S-PMA, microg/g creatinine) = 0.712 log (C(I)-benzene, ppm) + 1.644 (n = 145, r = 0.74, P < 0.001). The geometric mean (GSD) of S-PMA concentrations in urine from 45 subjects occupationally not exposed to benzene but smoking more than 20 cigarettes/day was 7.8 microg/g creatinine (2.11), the corresponding value among non-smokers being 1.0 microg/g creatinine (2.18). It is concluded that the urinary level of S-PMA can be regarded as a useful indicator of exposure to benzene.