Inhibitory effect of arbutin (hydroquinone-beta-D-glucopyranoside) on the melanogenesis was studied biochemically using cultured B16 melanoma cells. The maximum arbutin concentration lacking an inhibitory effect on cell growth was 5 X 10(-5) M. At this concentration, melanin content per cell was decreased significantly to about 39%, compared with that of arbutin untreated cells. Also, tyrosinase activity of arbutin treated cells was decreased significantly. When arbutin was added to B16 melanoma cell suspension, arbutin was not hydrolyzed to liberate hydroquinone. Further, tyrosinase activity in crude preparations from B16 melanoma cells was inhibited by arbutin. From these results, it is suggested that arbutin can inhibit the melanogenesis by affecting not only the synthesis but also the activity of tyrosinase rather than by killing melanocytes B16 melanoma cells. Also, it is suggested that hydroquinone is not responsible for the inhibitory effect of arbutin on the melanogenesis.