Benzene is a widely used solvent, currently present in the industrial environment at concentrations in the order of ppm. A valid method of biological monitoring that is easy to perform is needed for assessing occupational exposures. Benzene is metabolized in the body by microsomal cytochrome P-450 mono-oxygenase system into benzene epoxide. Benzene epoxide is metabolized along three different pathways which end in the excretion of trans, trans muconic acid, S-phenyl-mercapturic (S-PMA) and different phenols. A new method has been developed to evaluate urinary S-PMA of subjects exposed to benzene. Human urine is acidified with HCl to PH 1 and passed through a Sep-Pak C18 cartridge. The cartridges are washed with diluted HCl and a mixture of water/methanol/acetic acid and then eluted with acidified chloroform. The eluate is dried and reconstituted with a buffer phosphate, then passed through an anionic exchange cartridge (SAX) which is washed with diluted buffer and diluted HCl. S-PMA is recovered by eluting with concentrated buffer and is transformed into S-phenyl-cysteine. Finally, S-phenyl-cysteine is detected by HPLC connected with a fluorescence detector (wavelengths: excitation 330 nm, emission 440 nm) after derivatization with o-phthalaldehyde (OPA) and 2-mercapto-ethanol (MCE). The detection limit of the method is about 0.5 micrograms/l, the recovery of S-PMA is 90.0% and the variation coefficient is 3.8%. The method was checked on urine samples of 8 male non-smokers and 10 smokers: median values of 1.3 and 9.2 micrograms/g creatinine respectively of S-PMA were obtained. A further analysis on urine samples of 66 occupationally exposed workers (smokers and non-smokers) revealed a median value of S-PMA of 46.6 micrograms/g creatinine, compared with a median environmental benzene exposure of 1.99 mg/m3. These results suggest that S-PMA can be regarded in the future as a useful indicator for monitoring individual and collective low-level benzene exposure.