AIM To study the inhibitory effect of the cytotoxin (CTX) from guangxi cobra venom on human nasopharyngeal cancer cell (CNE) and other tumons cells. METHODS The cytotoxin was isolated and purified from Guangxi cobra venom by successive chromatography on Sephadex G-50 and CM Sepharose CL-6B columns. Its cytotoxic effects and does-effect relationship on human tumors cell lines were examined by MTT assay. RESULTS The inhibitory effects of CTX CM-5 on CNE, human ovarian carcinoma cell (Ho8990), uterus cervical carcinoma cell (HELA) and lymphoma (YAC) cell lines showed a definite does-effect relationship. The IC 50 (48 h in cubation) was 1.84, 2.59, 1.84 and 0.75 mg?L -1 respctively. The inhibitory effects of CTX CM-5 on CNE increased with time. The IC 50 (3 h and 24 h cubation) was 4.78 and 1.04 mg?L -1 respctively. CONCLUSION CTX from Guangxi cobra venom exhibites strong suppressive effect on cultured tumor cells line in vitro.