AbstractPhenol‐oxidizing enzymes such as peroxidases, laccases, and mushroom polyphenol oxidase are capable of catalyzing the oxidation of a wide range of phenolic pollutants. Although the use of these enzymes in waste‐treatment applications has been proposed by a number of investigators, little information exists on the toxicological characteristics of the oxidation products. The enzymes chloroperoxidase, horseradish peroxidase, lignin peroxidase, and mushroom polyphenol oxidase were used in this study to catalyze the oxidation of phenol, several mono‐substituted phenols, and pentachlorophenol. Seventeen reaction mixtures representing selected combinations of enzyme and parent phenol were subjected to mutagenicity screening using the AmesSalmonella typhimuriumplate incorporation assay; five selected mixtures were also incubated with the S9 microsomal preparation to detect the possible presence of promutagens. The majority of reaction mixtures tested were not directly mutagenic, and none of those tested with S9 gave a positive response. Such lack of mutagenicity of enzymatic oxidation products provides encouragement for establishing the feasibility of enzyme‐catalyzed oxidation as a waste‐treatment process. The only positive responses were obtained with reaction products from the lignin peroxidase–catalyzed oxidation of 2‐nitrophenol and 4‐nitrophenol. Clear positive responses were observed when strain TA100 was incubated with 2‐nitrophenol reaction‐product mixtures, and when strain TA98 was incubated with the 4‐nitrophenol reaction mixture. Additionally, 2,4‐dinitrophenol was identified as a reaction product from 4‐nitrophenol, and preliminary evidence indicates that both 2,4‐ and 2,6‐dinitrophenol are produced from the oxidation of 2‐nitrophenol. Possible mechanisms by which these nitratio
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