A known biotransformed compound, 6,7,4'-trihydroxyisoflavone, was identified as a potent tyrosinase inhibitor. It inhibited mushroom tyrosinase with an IC50 value of 9.2 microM, which is six times the anti-tyrosinase activity of kojic acid (IC50 = 54.4 microM). The inhibition kinetics, analyzed by Lineweaver-Burk plots, indicated 6,7,4'-trihydroxyisoflavone to be a competitive inhibitor of tyrosinase when L-tyrosine was used as a substrate. Its biosynthesis precursors and analogs, including glycitein, daidzein, and genistein, showed little anti-tyrosinase activity. The results suggest that hydroxyl groups at the C-6 and C-7 positions of the isoflavone skeleton might play an important role in the expression of tyrosinase inhibitory activity.