The effects of chronic treatment with hot water extract of Bitter Melon (Momordica charantia L.) or Ginger Rhizome (Zingiber offifinale Rosc) on spontaneous mammary tumorigenesis were examined in SHN virgin mice. In mice given free access to extract of Bitter Melon (0.5%) or Ginger (0.125%) in drinking water, the development of mammary tumors was significantly inhibited. Furthermore, the former inhibited uterine adenomyosis with a common pathological background to mammary tumors and the latterinhibited mammary tumor growth. While the mechanism of the effects of these natural products remains to be clarified, there were no adverse effects of chronic treatment with these agents as estimated from body weight, food and water intake and various plasma component levels as well as external appearance. Thus, these natural products, popular in Japan as foodstuffs, also appear to have a health benefit.