The inhibitory effects of several licorice flavonoids including liquiritin, licuraside, isoliquiritin and licochalcone A on the monophenolase activity of tyrosinase were studied. The results indicated that the IC_50 values of licuraside, isoliquiritin and licochalcone A were 0.072 mM,0.038 mM,0.0258 mM, respectively, and they are all competitive inhibitors. These flavonoids have the potential to be further developed into effective skin whitening agents. Different from the above flavonoids, no inhibitory act...