A common flavonol, kaempferol, isolated from the fresh flower petals of Crocus sativus L. (Iridaceae) was found to inhibit the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by mushroom tyrosinase with an ID(50) of 67 microgram/mL (0.23 mM). Interestingly, its 3-O-glycoside derivatives did not inhibit this oxidation. The inhibition kinetics analyzed by a Lineweaver-Burk plot found kaempferol to be a competitive inhibitor, and this inhibitory activity presumably comes from its ability to chelate copper in the enzyme. This copper chelation mechanism can be applicable for all of the flavonols as long as their 3-hydroxyl group is free. However, quercetin, kaempferol, and galangin each affect the oxidation of L-tyrosine in somewhat different ways.