Some guanidino compounds have been found to inhibit spontaneous mammary tumourigenesis in mice. In the present study, the effects of chronic treatment with Gold Banded Lily (Lilium auratum LindI) or Chinese Milk Vetch (Astragalus sinicus L), which contains L-arginine or L-canavanine, on spontaneous mammary tumourigenesis were examined in SHN mice. Free access to drinking water containing hot water extract of each natural product at a concentration of 0.5% significantly inhibited the development, but not growth, of mammary tumour. The mechanism of the protective role of each natural product in mammary tumours remains to be clarified; however, there were no significant long-term deleterious side-effects of chronic treatment with either product estimating from body weight change, food and water intakes and plasma component levels. Thus, the present findings suggest that these natural products containing guanidino compounds may act as prophylactic agents for mammary and other types of tumours.