A double-stranded RNA virus of the chestnut blight pathogen, Cryphonectria parasitica, has been shown previously to reduce accumulation of mRNAs of extracellular laccase (laccase A) produced by this fungus. Both extra- and intracellular laccases have been detected after growth of the fungus in liquid culture. In addition to cellular localization, the two laccases are distinguishable by time of appearance during growth and electrophoretic mobility. Laccase A was purified from the culture filtrate by standard protein purification procedures. The enzyme was characterized as a glycoprotein with a molecular mass of approximately 77 kDa. Both laccase A and laccase B activities were significantly reduced in the hypovirulent (double-stranded RNA-infected) strain UEP1 compared with the isogenic virulent (double-stranded RNA-free) strain EP155/2.