The antisense inhibitor poly-2'-O-(2,4-dinitrophenyl)-5'-GGCUGCGUGCCUCCUCACUGG (antisense poly-DNP RNA-21) has been synthesized by in vitro transcription followed by chemical derivatization. Its base sequence is complementary to that of nucleotides 110-130 in the mRNA of the regulatory RIalpha subunit of PKA (RIalpha/PKA), which is overexpressed in MCF-7 breast cancer cells and A549 lung cancer cells. The bioavailable and RNase-resistant antisense poly-DNP RNA-21 was found to inhibit cell growth with 50% inhibitory concentration (IC50) values of 0.05 nM in MCF-7 cells and 4 nM in A549 cells. The control 21-nt RNAs with the same poly-DNP oligonucleotide (ODN) platform but with scrambled, sense, or mismatched base sequence are inactive. Treatment of MCF-7 cells with antisense poly-DNP RNA-21 abolishes both the steady-state concentration of RIalpha mRNA and the synthesis of RIalpha protein. At sufficiently high concentration, antisense poly-DNP RNA-21 selectively kills the targeted cancer cells by inducing apoptosis. The observed sequence specificity and extremely low IC50 values of antisense poly-DNP RNA-21 suggest that it is a promising candidate for in vivo testing as an effective anticancer agent.