Harringtonine (HT), produced from Cephalotaxus species, is known to exhibit potent antiproliferative activity against myeloid leukemia cells by inhibiting protein synthesis. A previous study using acute promyelocytic leukemia (HL-60) cells raised the possibility that the C-5' methyl group of HT plays an important role in regulating leukemia cell line antiproliferative activity. In order to investigate the effect of hydrocarbon chains at C-5' on the resultant activity, the C-5' methyl group was replaced with various straight- and branched-chain hydrocarbons using the corresponding alcohols, and their antiproliferative activity against HL-60 and HeLa cells was investigated. As a result, 4'-n-heptyl-4'-demethylharringtonine (1f, n-heptyl derivative) showed the most potent cytotoxicity among the HT ester derivatives produced, with IC50 values of 9.4 nM and 0.4 μM for HL-60 and HeLa cells, respectively. Interestingly, the cytotoxicity of derivative 1f against HL-60 and HeLa cells respectively was ∼5 (IC50 = 50.5 nM) and ∼10 times (IC50 = 4.0 μM) those of HT and ∼2 (IC50 = 21.8 nM) and ∼4 times (IC50 = 1.7 μM) more than homoharringtonine (HHT). These results demonstrate the potential of the derivative 1f as a lead compound against leukemia.