To investigate the whitening activity of Laminaria japonicus, we measured the effect of hot-water extracts on melanin production in B16 melanoma cells. Melanocyte stimulating h ormone (MSH, $1{mu}M$)-stimulated melanin production in B16 melanoma cells was significantly inhibited by fucoidan and hot-water extracts, but not by alginate. The purified tyrosinase activity was not affected by hot-water extracts, alginate and fucoidan. However, tyrosinase expression was significantly inhibited by fucoidan and hot-water extract in Western blot. These results suggest that inhibitory mechanism of hot-water extracts on melanin production may be due to the inhibition of tyrosinase expression but not to direct inhibition of tyrosinase, such an effect of which may be dependent on fucoidan.