The biological activity of siRNA seems to be influenced by local characteristics of the target RNA, including local RNA folding. Here, we investigated quantitatively the relationship between local target accessibility and the extent of inhibition of the target gene by siRNA. Target accessibility was assessed by a computational approach that had been shown earlier to be consistent with experimental probing of target RNA. Two sites of ICAM-1 mRNA predicted to serve as accessible motifs and one site predicted to adopt an inaccessible structure were chosen to test siRNA constructs for suppression of ICAM-1 gene expression in ECV304 cells. The local target-dependent effectiveness of siRNA was compared with antisense oligonucleotides (asON). The concentration dependency of siRNA-mediated suppression indicates a >1000-fold difference between active siRNAs (IC50 approximately 0.2-0.5 nM) versus an inactive siRNA (IC50 > or = 1 microM) which is consistent with the activity pattern of asON when relating target suppression to predicted local target accessibility. The extremely high activity of the siRNA si2B (IC50 = 0.24 nM) indicates that not all siRNAs shown to be active at the usual concentrations of >10-100 nM belong to this highly active species. The observations described here suggest an option to assess target accessibility for siRNA and, thus, support the design of active siRNA constructs. This approach can be automated, work at high throughput and is open to include additional parameters relevant to the biological activity of siRNA.