Arbutin, hydroquinone-O-beta-D-glucopyranoside (1) was found to inhibit the oxidation of l-tyrosine (monophenolase activity) catalyzed by mushroom tyrosinase. However, arbutin itself was oxidized as a monophenol substrate at an extremely slow rate, and this oxidation was accelerated as soon as catalytic amounts (0.01 mM) of l-3,4-dihydroxyphenylalanine (L-DOPA) became available as a cofactor. The result observed was supported by monitoring oxygen consumption. The depigmenting mechanism of arbutin previously reported is supportable if a cofactor is not available in the melanocytes. The combination with L-ascorbic acid is a useful application, particularly when oxygen is limited.