Objective:The roots of two Thai medicinal plants,Clerodendr indicum and Clerodendrum villosum are found in traditional medicine practices.The aim of this research was to preliminarily study the cytotoxicity of extracts of their roots,and the parts that possessed cytotoxic activity were separated on a chromatograph to identify their active compounds.Methods:The extracts of both plants were screened for cytotoxicity on the SW620 cell line and the compounds isolated from the active extracts were further evaluated for their cytotoxic activity against five human cancer cell lines,including SW620,ChaGo-K-1,HepG2,KATO-Ⅲ and BT-474 using the 3-(4,5-dime thyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay.Results:Dichloromethane extracts of C.indicum and C.villosum were active against the SW620 cell line.Triterpenoids were mostly obtained from the extracts of these plants (0.28％ and 1.02％,respectively) and exhibited varying degrees of cytotoxicity and specificity against the tested cell lines.Two triterpenoids,oleanolic acid 3-acetate and betulinic acid,displayed moderate to strong cytotoxicity toward all cancer cell lines,with 50％ inhibitory concentration (IC50) values of 1.66-20.49 μmol/L,whereas 3β-hydroxyD:B-friedo-olean-5-ene and taraxerol were cytotoxic to only the SW620 cell line (IC50=23.39 and 2.09 μmol/L,respectively).Triterpenoid,lupeol,showed potent cytotoxicity on both SW620 (IC50=1.99 μmol/L) and KATO-Ⅲ cell lines (IC50 =1.95 μmol/L),while a flavonoid,pectolinarigenin,displayed moderate cytotoxicity against these cells (IC50 =13.05 and 24.31 μmol/L respectively).Although the widely distributed steroid,stigmasterol,was effective against the SW620 cell line (IC50 =2.79 μmol/L) and β-sitosterol was also active against SW620 (IC50 =11.26 μmol/L),BT-474 (IC50 =14.11 μmol/L) and HepG2 cancer cells (IC5o=20.47 μmol/L),none of the characteristic 24β-ethylsteroids of either Clerodendrum species were shown to be cytotoxic.Conclusion:This study is the first report on the presence of cytotoxic triterpenoids from the roots of these medicinal plants,which have been used in herbal formulas as an antipyretic.Our findings support further in-depth study of this pharmacological activity as an anticancer agent.