One of the biomarkers suggested by the ACGIH to assess the professional exposure to benzene is the S-phenylmercapturic acid in the end-shift urine. The existence in the urine of N-acetyl-S(1,2-dihydro-2hydroxypHenyl)-L-Cysteine, a precursor of SPMA that can be turned into it by acid hydrolysis, is a possible cause of miscorrelation between environmental and biological monitoring. The amount of measured SPMA depends on the degree of hydrolysis and therefore it is a function both of the urine PH and of the storage conditions of the sample. 40 urine samples have been collected from workers exposed to benzene, both smokers and not smokers, and for each sample the percentage of SPMA measurable at pH 2 and without pH correction (free SPMA) has been calculated with respect to the SPMA measured after quantitative hydrolysis, with the objectives to determine if a correct assessment of the exposure requires the determination of total SPMA and which concentration value could correspond to the BEI of 25 microg/g of creatinine established by the ACGIH. An aliquot of the urine samples has been treated with 9M H2SO4, a second one is brought to pH 2 and a third one is analyzed as it is. All samples are analyzed by HPLC/MS/MS in negative ions/MRM mode, and quantitative analysis is performed using the internal standard method. The percentage found in samples treated at pH 2 is on average 45% of the total SPMA for smokers and 60% for non smokers, while the free SPMA varies from 1% to 66% due to the urine pH variability and to the lower concentrations detected. The determination of total SPMA allows the standardization of the preanalyticalfactors and the dosage with analytical methods less sensitive than HPLC/MS/MS.