The potential toxicity of industrial wastewater, or effluent, was evaluated with a very rapid and sensitive chemiluminescent peroxidase assay. The assay, based on the peroxidase-catalyzed oxidation of luminol by hydrogen peroxide, is responsive to the presence of radical scavengers and enzyme-inhibiting substances. The chemical competition of the chemicals between DNA and the chemiluminescent peroxidase reaction was also investigated in order to evaluate the DNA-binding properties of these where DNA binding may lead to harmful adducts. The assay is rapid, requiring only 2 -5 min incubation with the test sample at 21ЊC. For the DNA competition assay, DNA was added to the test sample 15 min before the incubation step in order to evaluate reversal of light inhibition or protection of luminescence. Of the 21 industrial effluents studied, 76% had an inhibiting effect on light emission, suggesting that these effluents are probably toxic. Indeed, it was found that these effluents were toxic with at least one of the following: the Microtox assay, the rainbow trout lethality test, and Ceriopdaphnia dubia survival/ reproduction tests. It was found that this assay complied relatively well ( ) 60% with the Microtox toxicity test, suggesting that the light inhibiting effects of the effluent correspond to toxic effects in bacteria. The addition of DNA to the reaction mixture prevented inhibition of light ( ) emission i.e., DNA has a protective effect in 29% of the effluents tested, suggesting that these effluents contained potentially genotoxic chemicals. Moreover, DNA-reactive effluents proved to be genotoxic to ( ) ( ) Escherichia coli SOS Chromotest most of the time 70% , suggesting that DNA competition is also related to DNA damage. No effluent displayed genotoxicity with the SOS Chromotest and did not show DNA binding with the chemiluminescent peroxidase assay. The chemiluminescent peroxidase assay may be used for prescreening studies when the number of test samples is very high or when ( ) information about the potential geno toxicity is urgently needed.