PURPOSE: A carcinostatic agent that contains a phenylpolysilsesquioxane of a specific structual unit as an effective component, thus showing very low toxicity.

CONSTITUTION: The objective agent contains a phenylpolysilsesquioxane with a structural unit of the formula (R is phenylene; X is H, halogen, cano, alkoxy, phenoxy, alkyl, haloalkyl, carbamoyl, carboxyl, -COOM; M is alkyl, alkali metal, alkaline earth metal, ammonium) such as 4-carboxyphenylsilsesquioxane. An organic siloxane bond represented by 01.5Si-phenylene- is assumed to play an important role in the carcinostatic activity. The roxicity is very low againt warm blood animals and high carcinostatic activity is observed against tumor cells such as Ehrlich' or Walker's carcinosarcoma. It is administered orally or parenterally.