Harringtonine (HT) is a naturally occurring alkaloid isolated from the plant genus Cephalotaxus. It possesses antileukemic activity and has been clinically utilized for the treatment of acute leukemia and lymphoma. Sodium periodate (NaIO4) was reacted with HT to produce five HT derivatives including four novel compounds. Their antiproliferative activity against HL-60 acute promyelocytic leukemia cells revealed that the presence of the C-5' methyl group enhances the antiproliferative activity because the IC50 values of the HT derivatives, including HT1 (5'-de-O-methylharringtonine), were at least 2000 times higher (>100 μM) than that of HT (∼47 nM). In addition, an indirect competitive enzyme-linked immunosorbent assay (icELISA) using a monoclonal antibody against HT (mAb 1D2) revealed that these antiproliferative activities were related to their cellular uptake. These results indicated that esterification of HT1 at the C-4' carboxylic acid group may enhance the antiproliferative activity of HT.