The biological effects of the cysLTs, including LTC4, LTD4, and LTE4 are transduced mainly by a pair of 7-transmembrane receptors, CysLT1 and CysLT2.{7174.8519,10167} BAY u9773 is a non-selective antagonist of the CysLT receptors, having about the same IC50 (20-80 nM) for the inhibition of LT responses in a variety of tissue preparations containing either/or both receptors.{12765} In contrast, many of the commercially developed CysLT1 receptor antagonists (pranlukast or montelukast) antagonize only CysLT1 receptors. BAY u9773 is thus one of the only available tools for blocking CysLT2 receptors.