The inhibitory effects of a novel matrix metalloproteinase inhibitor, ONO-4817, on the development of mammary tumors and the progression of uterine adenomyosis were examined in SHN mice. First, multiparous mice which developed mammary tumors spontaneously were used. The first palpable tumor was removed, and the mice were thereafter fed chow containing ONO-4817. Any second mammary tumor developing in the other mammary fat pad was also removed, and the mice were continuously fed the chow containing ONO-4817. The mice were killed when a third tumor was detected in the other fat pad. The periods between the occurrence of the first and second tumors, and the second and third ones were significantly increased in the mice treated with ONO-4817 compared to the mice not given ONO-4817 treatment. Second, to test ONO-4817 suppression of the progression of the invasion of uterine adenomyotic tissue, mice with experimentally-induced adenomyosis were treated with ONO-4817 for 4 weeks. The degree of pathological progression of adenomyosis was less in the uteri exposed to ONO-4817 than in the uteri not exposed to the inhibitor.