Forsythiae Fructus was studied on cytotoxic activities for the purpose of finding out active consituents against L1210 and HL60 cells. To isolate the active ones, the methanolic extract was partitioned into water insoluble and water soluble fractions. Furthermore, the water soluble fraction was fractionated into four parts, n-hexane, benzene, ethylacetate and water fractions. Among these, the water insoluble fraction showed the most potent cytotoxic activities on L1210 and HL60 cells in vitro. The water insoluble fraction was applied to silica gel column chromatography and divided into 5 fractions(fr. 1-5). The active constituents I and II were isolated from fr.2 and 3, respectively, by repeated silica gel column chromatography and recrystallization. The constituents were identified as 3${eta}$-acetylbetulinic acid and betulinic acid by means of physicochemical data. The $ED_{50}$ values of 3${eta}$-acetylbetulinic acid and betulinic acid were 9.10 and 16.43${mu}g$/ml against L1210 cells and 2.72 and 2.41${mu}g$/ml against HL60 cells, respectively.