To study the inhibitory effects of caspase-3 mRNA antisense oligodeoxynucleotides (ASODNs) on apoptosis, we designed four ASODNs targeting different regions of caspase-3 mRNA and transfected them into human leukemia HL-60 cells. The transfected cells were given 10 Gy gamma-irradiation followed by incubation for 18 h and measurement of apoptosis and caspase-3 expression. Our results showed that ASODN-2 targeting the 5' non-coding region of sites -62 to -46, and ASODN-3 targeting the 5' coding region of sites -1 to 16, both reduced apoptosis measured by gel electrophoresis and flow cytometry. Hoechst 33258 staining and TUNEL assay revealed that apoptotic indexes in the ASODN-2 and ASODN-3 groups were significantly lower than those in the untransfected and mismatched oligodeoxynucleotide (MODN) groups. Immunocytochemistry, Western blotting and RT-PCR showed that expression levels of caspase-3 protein and mRNA in both ASODN-2 and ASODN-3 groups were decreased compared with those in the untransfected and MODN groups. In conclusion, caspase-3 mRNA ASODNs can inhibit gamma-radiation-induced apoptosis of HL-60 cells and reduce expression of caspase-3 protein and mRNA. The results suggest that antisense approach may be useful for therapeutic treatment of certain neurodegenerative diseases in which apoptosis is involved.