Phenolic compounds produced by sclerotia of Inonotus obliquits are the active constituents responsible for antioxidant activities. In this study, I. obliquus was grown in a continuously stirred tank reactor (CSTR) to explore how it accumulates phenolic compounds in different culture media and whether these compounds possess antioxidant activities. Phenolic compounds produced by I. obliquus in the control medium consisted of melanins, flavonoids, polyphenols and small phenolics. Their accumulation was affected by adding H(2)O(2) to the medium, where increased levels of total intracellular phenols (TIP) and melanins, but less total extracellular phenol (TEP) occurred. Simultaneous exposure to H(2)O(2) and arbutin resulted in a further increase in TIP production and reduced accumulation of TEP. Both TIP and TEP obtained at different culture ages and media were active in scavenging Superoxide anion and DPPH radicals. Therefore, production of phenolic Compounds by I. obliquus is enhanced by imposing oxidative stress, which might allow it to be exploited as a reliable Source of pharmaceutically important phenolic compounds. (C) 2008 Published by Elsevier Ltd.