To investigate the efficacy and the mechanism of the anti-inflammatory effect of Taraxacum officinale leaves (TOLs), the effect of a methanol extract and its fractions recovered from TOLs on lipopolysaccharide (LPS)-induced responses was studied in the mouse macrophage cell line, RAW 264.7. Cells were pretreated with various concentrations of the methanol extract and its fractions and subsequently incubated with LPS (1 microg/mL). The levels of nitric oxide (NO), prostaglandin (PG) E(2), and pro-inflammatory cytokines including tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6 were determined using enzyme-linked immunosorbent assays. Expressions of inducible NO synthase (iNOS) and cyclooxygenase (COX)-2 and activation of mitogen-activated protein (MAP) kinases were analyzed using western blotting. The methanol extract and its fractions inhibited LPS-induced production of NO, pro-inflammatory cytokines, and PGE(2) in a dose-dependent manner. The chloroform fraction significantly suppressed production of NO, PGE(2), and two pro-inflammatory cytokines (TNF-alpha and IL-1beta) in a dose-dependent manner with 50% inhibitory concentration values of 66.51, 90.96, 114.76, and 171.06 microg/mL, respectively. The ethyl acetate fraction also inhibited production of the inflammatory molecules. The chloroform and ethyl acetate fractions reduced LPS-induced expressions of iNOS and COX-2 and activation of MAP kinases in a dose-dependent manner. Among the fractions of the methanol extract, the chloroform and ethyl acetate fractions exhibited the most effective anti-inflammatory activities. These results show that the anti-inflammatory effects of TOLs are probably due to down-regulation of NO, PGE(2), and pro-inflammatory cytokines and reduced expressions of iNOS and COX-2 via inactivation of the MAP kinase signal pathway.