The effects of a selective antimetastatic agent: the aryldimethyltriazene derivative 1-p-(3,3-dimethyl-1-triazeno)benzoic acid potassium salt (DM-COOK) have been examined on two in vitro tumor cell lines derived from lung metastases of Lewis lung carcinoma. These stabilized in vitro tumor cell lines named C108 and BC215 have been reported to differ in their metastatic potential evaluated as lung colony forming ability and as the number of spontaneous metastases produced after intramuscular implant of tumor cells. The cytotoxic effect of DM-COOK in vitro was also compared with the one demonstrated by the structure-related compound 4-(3,3-dimethyl-1-triazeno)imidazole-5- carboxamide (DTIC) on the same variant lines. Survival curves show a different chemosensitivity of the two in vitro lines to the DM-COOK treatment, whereas no differences were detected between C108 and BC215 after exposure to DTIC. Moreover, DM-COOK and DTIC exhibit different trends of cell killing, implying different mechanisms of action for the two drugs. Results are discussed in view of the selective in vitro action of the aryldimethyltriazene derivative DM-COOK on cells which express a high metastatic potential.