Genetic polymorphism is an important factor of individual susceptibility to drugs or to toxic substances in environmental and occupational exposure. Although polymorphism is never the only responsible of a disease, it can modify both the level of risk for adverse effects and the levels of biomarkers after exposure to some toxic agents. In this paper two important groups of polymorphic enzymes, responsible of the detoxification of exogenous substances, were examined: paraoxonase-1, involved in the metabolism of some organophosphorus insecticides, and glutathione S-transferases, involved in the detoxification of numerous epoxide metabolites. Particularly the role of GSTT1 and GSTM1 polymorphism on the biological indicators of 1,3-butadiene has been evaluated. Moreover, the influence of delta-aminolevulinic acid dehydratase, enzyme involved in the sinthesys of the eme, on the biological indicators of exposure and effect to lead has been also examined.