The bark of Prunus yedoensis is used in antitussive medicines and in oral herbal formulations for inflammatory skin disorders. In the present study, we explored whether P. yedoensis bark extract (PYE) and its solvent partitioned fractions could modulate lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF)-a and interleukin (IL)-6 in vivo and in vitro. In addition, we examined the effect of PYE extract and its fractions on LPS-induced NF-jB and mitogen-activated protein kinase (MAPK) signaling in mouse peritoneal macrophages. Oral treatment of PYE decreased serum levels of TNF-a and IL-6 in LPS injected mice. PYE inhibited LPS-induced TNF-a and IL-6 in macrophages at the transcriptional level and also suppressed LPS-induced IjBa degradation and MAPK activation in vitro. Among the fractions, the chloroform fraction, which contains genistein, naringenin, sakuranetin, prunetin, and amygdalin, showed inhibitory effects at much lower concentrations than the water and ethyl acetate fractions. Taken together, our results indicate that PYE was able to inhibit LPS-induced expression of TNF-a and IL-6, the latter of which was more prominent. The effects of PYE on inflammatory cytokine synthesis may involve modulation of NF-jB and MAPK activation.