Respiratory syncytial virus (RSV) is one of the most common respiratory pathogens. Immoderate inflammation plays a great role in causing RSV-induced diseases. In the present study, watsonianone A, isolated from the fruit of Rhodomyrtus tomentosa (Ait.) Hassk, was found to show a good inhibitory effect on RSV-induced NO production, with a half-maximal inhibitory concentration of 37.2 +/- 1.6 mu M. nzyme-linked immunosorbent assay and fluorescence quantitative polymeraSe chain reaction analyses indicated that watsonianone A markedly reduced both mRNA and protein levels of tumor necrosis factor oc-, interleukin 6, and monocyte chemoattractant protein 1 in RSV-infected RAW264.7 cells. Mechanistically, watsonianone A inhibited nuclear factor kappa B (NF-kappa B) activation by suppressing IxBa phosphorylation. Further analysis revealed that watsonianone A activated the thioredoxin system and decreased intracellular reactive oxygen species (ROS) levels, which are closely associated with NF-KB activation in RSV-infected cells. These results reveal that watsonianone A can attenuate RSV-induced" inflammation via the suppression of ROS-sensitive inflammatory signaling.