The effect of siRNA against respiratory syncytial virus (RSV) was investigated in the cultured cells. MA104 cells were inoculated by RSV and transfected by different variants of siRNA against RSV phosphoprotein (P) mRNA or non-specific siRNA as a control. The inhibition of RSV was assessed by microscopically studying the cells, titrating the virus, and estimating viral RNA quantity in the culture supernatants by real-time polymerase chain reaction (PCR). The most potent variants of siRNA caused an up to 8-day delay of microscopically detectable syncytium generation to 8 days, an up to 280-fold decrease in viral titers, and an up to 40-fold reduction in viral RNA quantity in the supernatants, as compared to the controls (p < 0.001). RSV mRNA is a suitable target for siRNA-mediated RSV replication inhibition, promising an advance in the treatment of RSV infection.