A manganese peroxidase preparation from the white-rot fungus Nematoloma frowardii was found to be capable of releasing up to 17% (CO2)-C-14 from C-14-labelled synthetic humic substances. The latter were prepared from [U-C-14]catechol by spontaneous oxidative polymerization or laccase-catalysed polymerization. The extent of humic substance mineralization was considerably enhanced in the presence of the thiol mediator glutathione (up to 50%). Besides the evolution of (CO2)-C-14, the treatment of humic substances with Mn peroxidase resulted in the formation of lower-molecular-mass products. Analysis of residual radioactivity by gel-permeation chromatography demonstrated that the predominant molecular masses of the initial humic substances ranged between 2 kDa and 6 kDa; after treatment with Mn peroxidase, they were reduced to 0.5-2 kDa. The extracellular depolymerization and mineralization of humic substances by the Mn peroxidase system may play an important role in humus turnover of habitats that are rich in basidiomycetous fungi.