A system comprising laccase and a suitable phenol such as 4-hydroxybenzoic acid (HBA) or synthetic lignin (DHP) exhaustively peroxidized linoleic acid in acetate buffer. The presence of phenols in lignin was essential since an exhaustively methylated preparation of the same lignin did not support peroxidation. The peroxidation rate was greatly enhanced by Mn2+, which was oxidized to Mn3+ by laccase/HBA, whereas H2O2 inhibited strongly due to rapid reduction of Mn3+ by H2O2 with concomitant formation Of O-2. When acetate was replaced by Mn3+-Chelating oxalate or malonate, there was no change in peroxidation rates in the absence of Mn2+, whereas strong inhibition was observed in the presence of Mn2+. In case of malonate part of the inhibition was due to H2O2 formation as a result of Mn3+ reduction by malonate. These findings suggest that laccase may contribute to fungal lipid peroxidation in vivo thus expanding its role in the biodegradation of lignin and other recalcitrant aromatic compounds. (c) 2005 Elsevier Inc. All rights reserved.