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Culture supernatants of the white-rot fungus Trametes versicolor were found to contain Mn(III)-complexing agents able to effectively promote manganese-dependent peroxidase (MnP)-mediated oxidation of phenol red. The high molecular weight fractions of these supernatants contained carbohydrate polymers that functioned as effective Mn(III)-complexing agents. Gluconic and glucuronic acids were also found to be effective Mn(III)-complexing ligands capable of supporting MnP-mediated phenol red oxidation, as was the cellobionic acid formed from cellobiose by cellobiose:quinone oxidoreductase (CBQase) (EC 1.1.5.1). The formation of cellulose-derived sugar acid manganese-complexing agents by CBQase increased in the presence of wood pulp and cellulolytic enzymes. CBQase, while oxidizing cellobiose, reduced insoluble Mn(IV)O2 to Mn(II) or Mn(III) by a reaction whose rate is determined by the nature of the manganese-complexing agents present. A model is proposed to explain how oxidation of carbohydrate and reduction of MnO2 and quinones by CBQase may complement oxidation by MnP, thus promoting lignin biodegradation.