The success of antisense therapeutics is not predictable despite their widespread use in biotechnology and molecular medicine. The relationship between RNA structure and biological effectiveness is largely not understood; however, antisense RNA-mediated effects in vivo seem to be related to annealing kinetics in vitro. This study suggests that terminal unpaired nucleotides and overall flexibility of antisense RNA directed against the human immunodeficiency virus type 1 (HIV-1) are related to fast RNA-RNA annealing in vitro as well as to strong inhibition of virus replication in human cells. Annealing rate constants of computer-selected antisense RNA species approach the values for natural antisense RNA in the order of 10(6) M-1s-1. When considering the unfavorable stability in cellular extracts of antisense RNA species that were found to anneal fast in vitro, an antisense effect against HIV-1 in human cells was observed that was 10- to 10,000-fold stronger than that measured for species predicted to anneal slowly. A computer-supported structural design of antisense RNA can serve as a platform to determine RNA-RNA association in vitro and biological effectiveness in living cells.