The effectiveness of t,t-muconic acid (t,t-MA) has been investigated as indicator of benzene exposure in workers of an oil refinery. 196 blue collar workers that operate in the field and 25 white collar workers who work in administration have been studied. Workers had not chronic diseases; all workers resulted fit to work. The environmental monitoring, effected with personal and fixed samplers (141 samplings), has shown that benzene air levels were lower than 170 mcg/m3. Biological monitoring was performed with measurements of blood benzene, urinary t,t-MA and urinary phenols. Each worker has been questioned regarding smoking habit, consumption of food containing sorbic acid and non-occupational exposure to benzene. Values of biological index were: t,t-MA 88 +/- 94.1 mcg/g cr, phenols 17.7 +/- 2.9 mg/g cr, blood benzene 25.4 +/- 4.3 ng/l. No significant differences were found between blue and white collars respect t,t-MA, phenols, blood benzene, non-occupational exposure to benzene, smoking habit, consumption of food containing sorbic acid. A meaningful correlation was found between the t,t-MA and the ingestion of the number of foods containing sorbic acid (r = 0.87). The results confirm that the t,t-MA seems incapable to distinguish occupationally exposed workers, at low levels of benzene exposure, from unexposed.