Human metabolism of benzene involves pathways coded for by polymorphic genes. To determine whether the genotype at these loci might influence susceptibility to the adverse effects of benzene exposure, 208 Bulgarian petrochemical workers and controls, whose exposure to benzene was determined by active personal sampling, were studied. The frequency of DNA single-strand breaks (DNA-SSB) was determined by alkaline elution, and genotype analysis was performed for five metabolic loci. Individuals carrying the NAD(P)H:quinone oxidoreductase 1 (NQO1) variant had significantly twofold increased DNA-SSB levels compared to wild-type individuals. The same result was observed for subjects with microsomal epoxide hydrolase (EPHX) genotypes that predict the fast catalytic phenotype. Deletion of the glutathione S-transferase T1 (GSTT1) gene also showed a consistent quantitative 35-40% rise in DNA-SSB levels. Neither glutathione S-transferase M1 (GSTM1) nor myeloperoxidase (MPO) genetic variants exerted any effect on DNA-SSB levels. Combinations of two genetic polymorphisms showed the same effects on DNA-SSB as expected from the data on single genotypes. The three locus genotype predicted to produce the highest level of toxicity, based on metabolic pathways, produced a significant 5.5-fold higher level of DNA-SSB than did the genotype predicted to yield the least genotoxicity.