To develop new whitening agents from natural products， we screened 80 compounds derived from crude drugs in Kampo medicine in a melanin synthesis inhibition assay using murine B16 melanoma cells. The screen revealed that treatment with alisol B， a triterpene from Alismatis rhizoma， significantly decreased both melanin content and cellular tyrosinase activity in B16 cells. However， alisol B did not directly inhibit mushroom tyrosinase activity in vitro. Therefore， we investigated the mechanism underlying the inhibitory effect of alisol B on melanogenesis. Alisol B suppressed mRNA induction of tyrosinase and its transcription factor， microphthalmia-associated transcription factor (MITF). Furthermore， alisol B reduced the phosphorylation of CREB and maintained the activation of ERK1/2. These results suggest that the reduction in melanin production by alisol B is due to the downregulation of MITF through the suppression of CREB and activation of ERK and that alisol B may be useful as a new whitening agent.