Based on epidemiological and biological data, Laminaria, a brown kelp seaweed, is proposed as an important factor contributing to the relatively low breast cancer rates reported in Japan. Several possible mechanisms for the influence of Laminaria on breast cancer are proposed: Laminaria is a source of nondigestible fiber, thereby increasing fecal bulk and decreasing bowel transit time; it changes the posthepatic metabolism of sterols; it contains an antibiotic substance that may influence fecal ecology; it contains 1-3 beta glucan, which alters enzymatic activity of fecal flora; and it stimulates the host-mediated immune response. It is suggested that Laminaria may play a role in preventing either the initiation of breast cancer or its promotion by endogenous physiological factors.