Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) have been detected in many species and environments their bioresistance and toxicity being of great concern. PCDDs and PCDFs, or the predioxins and -furans, are formed from chlorophenols (CPs) by burning and pyrolysis by arcing and by photolysis. PCDDs and PCDFs have also been found in emissions from automobiles, municipal waste incinerators, and nickel and copper smelting. Peroxidases (POs), a group of heme-proteins, are found in many organs and organisms. They are exceptional enzymes because of low substrate specificity and multiple reaction mechanisms. This enzyme-catalyzed free radical reaction resembles reactions in pyrolysis, arcing, and photolysis. Halogenated phenols are among the peroxidase substrates, and phenolic substrates have been found to yield dibenzodioxin- and dibenzofuran like products. The question then arose whether CP's in peroxidase-mediated reactions could yield chlorinated dibenzodioxins and dibenzofurans. Since no-one has yet reported a biological formation of PCDDs and PCDFs the authors have analyzed the rpdoct mixture from the lactoperoxidase-catalyzed oxidation of some chlorophenols.