Abstract: ATMP, 4 p-(3,3-dimethyl-1-triazene) phenylsulfamide-5,6-dimethoxypyrimidine was first synthesized by Tianjin Institute of Medical and Pharmaceutical Sciences. Effects of ATMP have been studied in comparison with those of cyclophosphamide (CY) in mice bearing Lewis lung carcinoma. At the dosages of 10, 20, and 40 mg/kg/d, ATMP was shown to inhibit the number and the weight of spontaneous pulmonary metastasis but showed no significant effect on the primary turnor. Antimetastatic effects were also demonstrated by histological examination. On the contrary, CY 18 mg/kg/d ip obviously inhibited the growth of both primary tumor and pulmonary metastasis, whereas under relatively lower dosage, no effects on the tumors of both locations were observed. The effects of the two agents on the fractional incorporation of ³H-TdR in tumor cells further indicated that at the dosage used in the experiment only ATMP was devoid of cytotoxic action for the primary tumor. The experiments also showed that ATMP did not modify the activity of Fc receptor of peritoneal macrophage in mice and did not reduce artificial metastasis. In the host pretreated with ATMP, no reduction of the formation of spontaneous metastasis was found.These data indicate that ATMP, differing from CY, has selective antimetastatic effects. It acts directly on tumor cells and presumably inhibits their release from primary tumor into the bloodstream.