The effects of quercetin on the activity of mushroom tyrosinase were studied. The equilibrium constants for this inhibitor binding with the enzyme molecule were established. The inhibition mechanism obtained from Lineweaver-Burk plots show that quercetin is a competitive inhibitor. In the time course of the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by the enzyme in the presence of different concentrations of quercetin, the rate decreased with increasing time until a straight line was approached. The inhibition of tyrosinase by quercetin is a slow and reversible reaction with residual enzyme activity. The microscopic rate constants were determined for the reaction of quercetin with the enzyme.