The medicinal mushroom Agaricus blazei produced high amounts of laccase (up to 5,000 units l(-1)) in a complex, agitated liquid medium based on tomato juice, while only traces of the enzyme (< 100 units l(-1)) were detected in synthetic glucose-based medium. Purification of the enzyme required three chromatographic steps, including anion and cation exchanging. A. blazei laccase was expressed as a single protein with a molecular mass of 66 kDa and an isoelectric point of 4.0. Spectroscopic analysis of the purified enzyme confirmed that it belongs to the "blue copper oxidases". The enzyme's pH optimum for 2,6-dimethoxyphenol (DMP) and syringaldazine was pH 5.5; but for 2,2'-azino-bis(3-ethylthiazoline-6-sulfonate) (ABTS) no distinct pH optimum was observed (highest activity at the lowest pH tested). Purified laccase was stable at 20 degrees C, pH 7.0 and pH 3.0, but rapidly lost its activity at 40 degrees C or pH 10. Sodium chloride strongly inhibited the enzyme activity, although the inhibition was completely reversible. The following kinetic constants were determined (K-m, k(cat)): 63 mu M, 21 s(-1) for ABTS, 4 mu M, 5 s(-1) for syringaldazine, 1,026 mu M, 15 s(-1) for DMP and 4307 mu M, 159 s(-1) for guaiacol. The results show that-in addition to the wood-colonizing white-rot fungi - the typical litter-decomposing basidiomycetes can also produce high titers of laccase in suitable liquid media.