An n-hexane extract of the roots of Aralia cordata Thunb. (Araliaceae) was found to show significant in vitro cytotoxic activity against P388D$_{1}$ lymphocytic leukemia cell in culture. Bioactivity-directed fractionation of this extract led to the isolation of four polyacetylenes, falcarindiol (1), dehydrofalcarindiol (2), falcarindiol-8-acetate (3) and dehydrofalcarindiol-8-acetate (4). Cytotoxicity of compounds 1 and 3 was found to be better than that of compounds 2 and 4 when these compounds were tested against eight in vitro tumor cell lines, namely, A549, HCT15, DLD1, MCF7, SKOV3, HL60, K562 and P388D$_{1}$. The fact that the cytotoxicity of compounds 1 and 3 against series of tumor cell lines was much stronger than that of compounds 2 and 4 suggested that the saturated carbon chain at the termial and the hydroxyl group at the C-3 are important for the activities. The requirement for the activity was further confirmed by synthesizing and assaying the acetate derivatives of compounds 1 and 2.