Preparations from the gum of Boswellia spec. have been used in the traditional medicine for the treatment of inflammatory diseases. Extracts from B. serrata gum were shown to inhibit leukotriene biosynthesis by impairing the 5-lipoxygenase (5-LO) activity. In order to identify the minimal effective concentrations of extracts in vitro we studied the effects of ethanolic extracts from commercially available resins from two regions (B. serrata gum from India and Olibanum in granis from Arabia) on the 5-LO product formation from endogenous substrate in calcium and ionophore stimulated neutrophils in a defined concentration range. Both extracts inhibited 5-LO product formation in vitro in concentrations greater than 10 to 15 μg/ml as reported previously for an ethanolic B. serrata extract. In contrast, lower concentrations of extracts (1 to 10 μg/ml) even potentiated 5-LO product formation, especially the biosynthesis of 5(S)-HETE. The in vitro data underline the major importance of drug standardization when Boswellia resin containing preparations are used for the treatment of diseases. AKBA:acetyl-11-keto-β-boswellic acid BA:β-boswellic acid 5-oxo-ETE:5-oxo-6,8,11,14-E,Z,Z,Z-eicosatetraenoic acid 5(S)-HETE:5(S)-hydroxy-6,8,11,14-E,Z,Z,Z-eicosatetraenoic acid 5-LO:5-lipoxygenase LT:leukotriene PGB2:prostaglandin B2 PMNL:polymorphonuclear leukocytes