We evaluated the antagonist activity of ONO-1078 against peptide leukotrienes (LTs) by a radioligand binding assay and functional experiments in guinea pigs. In the radioligand binding assay, ONO-1078 inhibited [3H]LTD4 and [3H]LTE4 bindings to lung membranes (Ki = 0.99 and 0.63 nM, respectively) and was 2,000- to 3,000-fold more potent than FPL55712. Antagonism of ONO-1078 against [3H]LTC4 binding (Ki = 5640 nM) was approximately twofold more potent than that of FPL55712. The antagonism of ONO-1078 against [3H]LTD4 binding was competitive. In functional experiments, ONO-1078 showed competitive antagonism against the LTC4- and LTD4-induced contractions of guinea pig trachea and lung parenchymal strips with a pA2 range of 7.70 to 10.71 and was approximately 400- to 3,300-fold more potent than FPL55712. Interestingly, in the presence of an inhibitor of the bioconversion of LTC4 to LTD4, ONO-1078 also antagonized the LTC4-induced contraction of guinea pig trachea (pA2 = 7.78). ONO-1078 significantly reversed the LTD4-induced prolonged contraction without effect on the KCl- and BaCl2-induced contractions of guinea pig trachea. Furthermore, ONO-1078 antagonized the antigen-induced SRS-A mediated contraction of guinea pig trachea. On the other hand, ONO-1078 showed no antagonism against histamine, acetylcholine, 5-hydroxytryptamine, prostaglandin D2 and U-46619. In addition, ONO-1078 showed little or no effect on the activities of cyclooxygenase, 5-lipoxygenase and thromboxane synthetase. These in vitro studies indicate that ONO-1078 is a highly potent, selective and competitive antagonist of peptide leukotrienes that acts with higher affinity at LTD4 and LTE4 receptors than LTC4 receptors.