Abstract: A cyclic tetrapeptide having the formula (I): ##STR1## wherein n is 4 or 3 and a process for preparing the above cyclic tetrapeptide. The cyclic tetrapeptide of the present invention inhibits transformation of cells caused by oncogenes and possesses activities for recovering to a normal cell and for inhibiting proliferation of carcinoma cells. Therefore, the cyclic tetrapeptide of the present invention is useful for an active ingredient of an antitumor agent.