Lignin degradation by Pleurotus ostreatus was studied under solid-state fermentation (SSF) in chemically defined medium containing various levels of Mn. Degradation of [14C]lignin prepared from cotton branches to soluble products, as well as its mineralization to 14CO2, was enhanced by the addition of Mn. The effect of malonate on lignin mineralization was most marked during the first 10 days of SSF, in a treatment amended with 73 μM Mn. A high concentration of Mn (4.5 mM) caused inhibition of both fungal growth and mineralization rates during the first 2 weeks of incubation. Addition of malonate reversed this effect because of chelation of Mn. Mn was found to precipitate in all treatments, with or without the addition of malonate. α-Keto-γ-methiolbutyric acid cleavage to ethylene, an indication of OH production, was observed as early as 3 days of incubation in all treatments.