Oxidation of spruce milled-wood lignins in colloidal state catalyzed by horseradish peroxidase (HRP) and lignin peroxidase (LiP) in the presence of H2O2 was compared in nonbuffered dimethylfor mamide/water. Lignins were characterized for their hydrodynamic properties by size-exclusion chromatography and structural bonding pattern by analysis after thioacidolysis. In contrast to LiP, HRP did not induce modification of lignin hydrodynamic properties. The lignin content in beta-O-4-linked guaiacyl monomers and dimeric structures, however, decreased after oxidation by the two enzymes, indicating large structural changes in the polymer. Preferential degradation by both LiP and HRP of beta-5 and beta-1 lignin dimeric units was also observed. This suggests a greater susceptibility to enzyme oxidation, as compared to the 5-5’ and 4-O-5 lignin substructures. In conclusion, the action of HRP on the lignin polymer. is similar in many respects to that of fungal LiP, but is distinctly different in its inability to cause a net destructuring of the macromolecular three-dimensional network.