The activity of the enzyme 7-ethoxy-resorufin-O-deethylase (EROD) has been extensively employed in biomonitoring studies of persistent organic pollutants (POPs) for more than a decade. Although the procedure is simple, convenient, sensitive and accurate. The cytochrome P450 monooxygenase 1A (CYP1A) is induced by several planar toxic compounds and endogenous chemicals, and the induction of this protein is often measured in terms of EROD activity. This protocol describes how to use EROD activity in the prediction of toxicity of chemicals in several models. The method can be employed in vitro and in vivo to assess the effects of drugs and toxic compounds on CYP1A1 enzyme. In aquatic biota EROD activity is a sensitive biomarker of exposure to certain planar halogenated hydrocarbons and the other structurally similar compounds. This method is rapid and the whole procedure takes no longer than 30 min including reagent preparation.