Saiboku-To, a mixture of extracts from 10 medicinal herbs, has been used for the treatment of bronchial asthma in Japan. Inhibitory action of this drug on arachidonate 5-lipoxygenase (5-LO) metabolism in rat basophilic leukemia cells (RBL-1 cells) was examined. Saiboku-To significantly inhibited calcium ionophore-stimulated synthesis of cysteinyl leukotrienes (cLTs) and leukotriene B 4 (LTB 4). Inhibition appeared 10 min after addition of the substance and reached a maximal value after 3 h. Saiboku-To did not inhibit the release of [ 3H]arachidonic acid (AA) from cell membrane by calcium ionophore stimulation, or the production of cLTs and LTB 4 when LTA 4-free acid was used as the substrate. However, it significantly inhibited the production of cLTs and LTB 4 when free AA was used as the substrate. The production of thromboxane A 2 (TXA 2), a cyclooxygenase metabolite, was not inhibited when AA was used as the substrate in cell free study. These results indicate that Saiboku-To selectively inhibits 5-LO activity in the metabolic pathway of AA.