The effective target sites for antisense oligodeoxynucleotides (AS-ODN) on bcl-2 mRNA, except its start region, were looked for and their effects on the sensitivity of HL-60 and K562 leukemia cells to arabinosyl cytosine (Ara-C) were observed. The secondary structure of bcl-2 mRNA was simulated with RNAstructure microsoftware, and AS-ODNs, targeting at some sites, were designed and synthesizd with phosphorothioate modifying. The median inhibitory concentration (IC(50)) of Ara-C for HL-60 and K562 cells was determined with MTT method; the apoptosis rate and bcl-2 protein expression level were assayed by flow cytometry. The results showed that 10 micro mol/L bcl-2 AS-ODN combined with Ara-C inhibited the expression of bcl-2 protein, increased apoptosis in HL-60 and K562 cells and decresed IC(50) of Ara-C significantly. The AS-ODN targeting the coding region of bcl-2 mRNA had stronger effect than AS-ODN targeting the translation initiation region. In conclusion, the much more effective sites for antisense oligodeoxynucleotides to target, except translation start region, might provide a new useful way for antisense drug design.