2,4,2',4'-Tetrahydroxy-3-(3-methyl-2-butenyl)-chalcone (TMBC), a naturally occurring compound from Morus nigra, modulated melanogenesis by inhibiting tyrosinase. TMBC inhibited the L-dopa oxidase activity of mushroom tyrosinase with an IC(50) value of 0.95+/-0.04 microM, which was more potent than kojic acid (IC(50)=24.88+/-1.13 microM), a well-known tyrosinase inhibitor. The kinetic studies of tyrosinase inhibition revealed that TMBC acts as a competitive inhibitor of mushroom tyrosinase with L-dopa as the substrate. Furthermore, TMBC effectively inhibited both cellular tyrosinase activity and melanin biosynthesis in B16 melanoma cells without significant cytotoxicity. The inhibitory effect of TMBC on melanogenesis was attributed to the direct inhibition of tyrosinase activity, rather than the suppression of tyrosinase gene expression. These results indicated that TMBC may be a new promising pigmentation-altering agent for cosmetic or therapeutic applications.