The antitumor and antimetastatic effects of p-(3-methyl-1-triazeno)benzoic acid potassium salt (MM-COOK) as compared with those of the parent 3,3-dimethyl derivative (DM-COOK) were examined using Lewis lung carcinoma, MCa mammary carcinoma of the CBA mouse and TLX5 lymphoma. Similarly to DM-COOK, MM-COOK reduces metastasis formation and significantly prolongs the survival of mice bearing the Lewis lung carcinoma when given at a daily dose corresponding to one-half that of DM-COOK. Unlike DM-COOK, MM-COOK exhibits significant cytotoxicity to metastatic foci and pronounced inhibition of primary tumor development. MM-COOK also causes cytotoxic effects on TLX5 lymphoma cell growing in the peritoneal cavity, even when used at low doses. The antimetastatic effects observed in mice bearing MCa mammary carcinoma are unrelated to the inhibition of primary tumor growth and are more likely due to the selection of clones endowed with lower metastatic ability. It appears that MM-COOK exhibits the same antineoplastic activity as DM-COOK, but the former does so at a lower daily dose and produces interesting cytotoxic effects other than those reflecting its antimetastatic properties. It thus seems to be a valid alternative to DM-COOK, in view of the possible introduction of newer aryltriazenes into clinical use.