PURPOSE: To provide a low-toxic and stable agent for increasing antitumor effect, by using 3'-deoxyguanosine-5'-monophosphate or 3'-deoxyadenosine-5'-monophos- phate as an active component.

CONSTITUTION: 3'-Deoxyguanosine-5'-monophosphate or 3'-deoxyadenosine-5'- monophosphate or their pharmacologically permissible salt is used in combination with radiotherapy and/or therapy with an antitumor agent for the purpose of increasing the antitumor effect. The active component inhibits the restoring function of a stationary cell affected with especailly severe latent lathal damage, and accordingly, enables the radiotherapy with reduced dose.