The induction of hydroxyl radical ( ⅐ OH) production via quinone redox cycling in white-rot fungi was investigated to improve pollutant degradation. In particular, we examined the influence of 4-methoxybenzaldehyde (anisaldehyde), Mn 2؉ , and oxalate on Pleurotus eryngii ⅐ OH generation. Our standard quinone redox cycling conditions combined mycelium from laccase-producing cultures with 2,6-dimethoxy-1, 4 generated during O 2 ⅐ ؊ reduction. Finally, induction of ⅐ OH production through quinone redox cycling enabled P. eryngii to oxidize phenol and the dye reactive black 5, obtaining a high correlation between the rates of ⅐ OH production and pollutant oxidation.The degradation of lignin and pollutants by white-rot fungi is an oxidative and rather nonspecific process based on the production of substrate free radicals (36). These radicals are produced by ligninolytic enzymes, including laccase and three kinds of peroxidases: lignin peroxidase, manganese peroxidase, and versatile peroxidase (VP) (23). The H 2 O 2 required for peroxidase activities is provided by several oxidases, such as glyoxal oxidase and aryl-alcohol oxidase (AAO) (9, 18). This free-radical-based degradative mechanism leads to the production of a broad variety of oxidized compounds. Common lignin depolymerization products are aromatic aldehydes and acids, and quinones (34). In addition to their high extracellular oxidation potential, white-rot fungi show strong ability to reduce these lignin depolymerization products, using different intracellular and membrane-bound systems (4, 25, 39). Since reduced electron acceptors of oxidized compounds are donor substrates for the above-mentioned oxidative enzymes, the simultaneous actions of both systems lead to the establishment of redox cycles (35). Although the function of these redox cycles is not fully understood, they have been hypothesized to be related to further metabolism of lignin depolymerization products that require reduction to be converted in substrates of the ligninolytic enzymes (34). A second function attributed to these redox cycles is the production of reactive oxygen species, i.e., superoxide anion radicals (O 2 ⅐ Ϫ ), H 2 O 2 , and hydroxyl radicals ( ⅐ OH), where lignin depolymerization products and fungal metabolites act as electron carriers between intracellular reducing equivalents and extracellular oxygen. This function has been studied in Pleurotus eryngii, whose ligninolytic system is composed of laccase (26), VP (24), and AAO (9). Incubation of this fungus with different aromatic aldehydes has been shown to provide extracellular H 2 O 2 on a constant basis, due to the establishment of a redox cycle catalyzed by an intracellular aryl-alcohol dehydrogenase (AAD) and the extracellular AAO (7, 10). The process was termed aromatic aldehyde redox cycling, and 4-methoxybenzaldehyde (anisaldehyde) serves as the main Pleurotus metabolite acting as a cycle electron carrier (13). A second cyclic system, involving a cellbound quinone reductase activity (QR) and laccase, was found to prod...