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The DNA-damaging ability of benzene and its metabolites on peripheral blood mononuclear cells (PBMC) has been investigated by using the alkaline comet assay. The PBMC were incubated with different compounds in two different media for 2 and 24 hr at concentrations that did not affect cell viability and the DNA damage was quantified by a computerized image analysis system. Benzene and phenol (5 mM) did not show any genotoxic activity after 2 hr of incubation in the two media tested, phosphate-buffered saline (PBS) and RPMI containing 5% of heat-inactivated fetal calf serum (RPMI + 5% FCS), whereas phenol was genotoxic and cytotoxic at 10 mM after 24 hr of incubation in RPMI + 5% FCS. All other benzene metabolites were genotoxic at micromolar concentrations when incubated in PBS with the following decreasing order of potency: benzenetriol, catechol, hydroquinone, and benzoquinone. When the PBMC were incubated in RPMI + 5% FCS, the effect of catechol (200-600 microM) and benzenetriol (10 microM) was reduced, whereas the genotoxicity of benzenetriol at high concentrations (50-100 microM) and hydroquinone (150-2500 microM) was not affected. In contrast, the effect of benzoquinone at 5 and 10 microM was greatly enhanced when the cells were incubated in RPMI + 5% FCS. This effect resulted mainly from the presence of serum in the medium and it was almost completely inhibited by boiling the serum (100 degrees C, 5 min) and was partially reduced by extensive dialysis. Benzoquinone was the most damaging compound when tested under more physiological conditions, thereby supporting the general observation that it is the most myelotoxic benzene metabolite., (Copyright 2001 Wiley-Liss, Inc.)