In the biobleaching of kraft pulp with white-rot fungus Phanerochaete sordida YK-624. it was confirmed that manganese dioxide (Mn(IV)) in kraft pulp was reduced to Mn(II) by ESR analysis. By this reaction. the production of manganese peroxidase (MnP) by P. sordida YK-624 was triggered and then the kraft pulp was bleached with MnP. In the liquid culture, the reduction of manganese dioxide was observed by the washed mycelia of P. sordida YK-624 although smaller amount of Mn(II) was produced by the extracellular culture fluid. The reduction by the washed mycelia was dependent on Fe(II), a metal chelator, and NAD(P)H. Moreover, the cell-free extract from P. sordida YK-624 catalyzed the reduction of ferric chelate to ferrous chelate in the presence of NAD(P)H. It is likely, therefore, that ferrous chelate reduces manganese dioxide and that the produced ferric chelate is reduced by intracellular NAD(P)H-dependent reductase to regenerate ferrous chelate. An intracellular NADPH-dependent reductase. namely NADPH-dependent ferrireductase (molecular Weight 35 kDa, pI 5.1), was purified from P. sordida YK-624. Moreover, there was a good correlation between the production of intracellular ferrireductase and that of extracellular MnP in a liquid culture containing manganese dioxide. These results suggest that the ferrireductase is involved in the production of MnP by the reduction of manganese dioxide to Mn(II).