RSV L-protein-IN-1 (compound D) is a potent inhibitor of Respiratory syncytial virus (RSV) (EC50=0.021 μM). RSV L-protein-IN-1 inhibits Polymerase (IC50=0.089 μM),and blocks RSV mRNA synthesis by inhibiting guanylation of viral transcripts. RSV L-protein-IN-1 shows moderate cytotoxicity (CC50=8.4 μM,HEp-2),also exhibits activity and lowers virus titers in mouse models of RSV infection[1].