Mulberroside A, a glycosylated stilbene, was isolated and identified from the ethanol extract of the roots of Morus alba. Oxyresveratrol, the aglycone of mulberroside A, was produced by enzymatic hydrolysis of mulberroside A using the commercial enzyme Pectinex. Mulberroside A and oxyresveratrol showed inhibitory activity against mushroom tyrosinase with an IC(50) of 53.6 and 0.49 microM, respectively. The tyrosinase inhibitory activity of oxyresveratrol was thus approximately 110-fold higher than that of mulberroside A. Inhibition kinetics showed mulberroside A to be a competitive inhibitor of mushroom tyrosinase with L-tyrosine and L-DOPA as substrate. Oxyresveratrol showed mixed inhibition and noncompetitive inhibition against L-tyrosine and L-DOPA, respectively, as substrate. The results indicate that the tyrosinase inhibitory activity of mulberroside A was greatly enhanced by the bioconversion process.