A review of the biological properties of seaweed is presented and the role of seaweed as a breast cancer anticarcinogen is suggested. Proposed mechanisms of action are: reduction of plasma cholesterol, binding of biliary steroids, inhibition of carcinogenic fecal flora, binding of pollutants, stimulation of the immune system, and the protective effects of beta-sitosterols. In an experiment using sarcoma-180 in mice, seaweed extract appeared to have an antitumor effect. Thus it is suggested that breast cancer may be prevented and that this dietary habit among the Japanese could be an important factor in understanding the lower breast cancer rates reported in Japan.