LTC4 is the parent cysLTs produced by the LTC4 synthase-catalyzed conjugation of glutathione to LTA4. LTC4 is produced by neutrophils, macrophages, and mast cells, and by transcellular metabolism in platelets. It is one of the constituents of slow-reacting substance of anaphylaxis (SRS-A) and exhibits potent smooth muscle contracting activity. LTC4-induced bronchoconstriction and enhanced vascular permeability contribute to the pathogenesis of asthma and acute allergic hypersensitivity. The concentration of LTC4 required to produce marked contractions of lung parenchymal strips and isolated tracheal rings is about 1 nM. LTC4 methyl ester is a more lipid soluble form of LTC4. The biological activity of LTC4 methyl ester has not been reported.