Menyanthes trifoliata L. is used in Swedish traditional medicine for the treatment of inflammatory diseases of the kidney, e.g. glomerulonephritis. Earlier studies have shown that MtL increases glomerular filtration rate after renal reperfusion ischemia. This activity was suggested to be PAF-inhibitory since MtL also inhibited PAF-induced exocytosis in vitro on human neutrophils (IC(50) = 0.16 mg/ml). The present study further characterizes the anti-inflammatory properties of a rhizome decoction of this plant. MtL inhibited carrageenan-induced rat paw edema (ID(50) ≈ 1.7 g/kg p.o.) and ethyl phenylpropiolate-induced rat ear edema (32% at 2.0 g/kg p.o.) in a dose-dependent manner. Further studies revealed that MtL inhibited both fMLP-induced exocytosis (IC(50) = 0.16 mg/ml) and elastase activity (IC(50) = 0.16 mg/ml). According to these results it is likely that the activity shown in the PAF-test is at least partly due to an inhibition of elastase. MtL showed only minor hemolytic properties at the concentrations used in the PAF- and fMLP-tests, suggesting that the cells in these tests are undamaged. The decoction also inhibited the biosynthesis of LTB(4) (IC(50) ≈ 0.73 mg/ml) and prostaglandins (IC(50) = 0.37 mg/ml) in vitro in a concentration-dependent way. However, at concentrations where the decoction is active in the LTB(4)-test, it also possesses hemolytic properties.