The toxic naphthoquinone juglone (5-hydroxy-1,4-naphthoquinone) is efficiently degraded by the ligninolytic fungus Pleurotus sajor-caju, as demonstrated by the total bleaching within 9 d of a conventional liquid culture medium supplemented with 0.6 mM juglone. The oxidative degradation involves the production of hydrogen peroxide arising from both enzymic and non-enzymic oxidation reactions, promoted by the fungus. Juglone is not directly attacked by the oxidative enzymes of the ligninolytic machinery of P. sajor-caju, such as laccase, manganese peroxidase and arylalcohol oxidase. On the other hand, this naphthoquinone is a good substrate for a reductase, which triggers an auto-oxidative process producing reactive oxygen species and leading to juglone degradation. The degradation process continues to completion by means of a direct, presumably non-catalysed reaction with hydrogen peroxide.