YM158, an orally active dual antagonist for LTD₄ and TXA₂ receptors, is expected to have a stronger antiasthmatic efficacy in a broader class of asthmatic patients than single antagonistic drugs.The effect of YM158 is examined on these asthmatic responses in mediator-controlled and passively sensitized guinea pigs. Because the inhibitory effects of YM158 on increase in the airway resistance induced by LTD₄ or U46619 are shown to be dose-dependent when p.o. administered 1 h before LTD₄ or U46619 injection, with ED₅₀ values of 8.6 and 14 mg/kg, respectively, the antagonistic activities of p.o. YM158 for LTD₄ and TXA₂ receptors are exhibited at the same dose range. Oral YM158 shows significant effects, approximately the same as the combination of Pranlukast and Daltroban on antigen-induced response under various conditions; namely, where LTD₄ is predominant, TXA₂ is predominant; or where both mediators participated equally. In groups not treated with Indomethacin, administration of Daltroban (10 mg/kg), a combination of Pranlukast (30 mg/kg) and Daltroban (10 mg/kg), or YM158 (30 mg/kg) significantly prolongs the onset time for asthmatic response and significantly suppresses symptoms^[2].