Antitumor polyoxomolybdates have been recognized in the course of study on the medical utilization of polyoxometalates, inorganic polymers of metal oxide. [NH3Pri]6[Mo7O24].3H2O (PM-8) was found as a representative of antitumor polyoxomolybdates. The growth suppressions of PM-8 against Co-4 human colon cancer xenografted under the subrenal capsule in cd-1 mice were equal or superior to that of 5-FU, MMC, ACNU, ADM and CDDP. Potent antitumor activity of PM-8 is also established against MX-1 human breast and OAT human lung cancer xenografted in athymic nude mice. Polyoxomolybdate is a new type of antitumor substance.