Small interfering RNAs (siRNA) could be the ideal inhibitor of specific gene expression if they could be delivered efficiently to their targets. Although siRNAs cannot by themselves cross cell membranes, they can enter cells with the help of transfection reagents via facilitated transport. Inside cells, however, they can still be hydrolyzed by cytoplasmic RNases before reaching their target. Here we show that poly-2'- O-(2,4-dinitrophenyl)-siRNA (DNP-siRNA) can by itself diffuse into mammalian cells and stay in the cells for several days, with no detectable degradation. Using the DNP-RNA targeting to the insulin-like growth factor receptor (IGF-IR) as an example, we demonstrate that the efficacy of this DNP-siRNA for inhibiting the growth of three different types of tumor cells is several-fold higher than that of the corresponding native siRNA. Similar results were obtained from Western blotting assay. As expected, the control siRNAs with mismatched, scrambled, and reverse sequences are all inactive. These results illustrate that both the biomembrane permeability and intracellular stability of siRNA can be greatly improved by DNP derivatization without loss of its sequence specificity.