The immunostimulatory effects of an extract from dried nori (Porphyra yezoensis) were examined in vitro and in vivo. In vitro, the nori extract induced the production of interferon-gamma (IFN-gamma) in murine spleen cells, while it induced IFN-gamma production and increased the cytotoxic activity for the human natural killer (NK) cells. In vivo, IFN-gamma production was induced in the serum of normal mice by a single oral administration of nori extract, and survival was prolonged in allograft mouse models of melanoma after multiple oral administrations of nori extract. These results demonstrate that the nori extract had immunostimulatory effects in vitro and in vivo.