11-trans LTC4 is a C-11 double bond isomer of LTC4. LTC4 undergoes slow temperature-dependent isomerization to 11-trans LTC4 during storage. 11-trans LTC4 is produced in smaller amounts relative to LTC4 in ionophore-stimulated HMC-1 cells (a human mast cell line) and equine eosinophils, but not in human neutrophils or RBL-1 cells. It is nearly equipotent with LTC4 for contraction of guinea pig parenchymal and ileum. In a radioligand binding assay using guinea pig ileum as a cysteinyl leukotriene receptor preparation, the pKis for LTC4 and 11-trans LTC4 were determined to be 6.42 and 6.58, respectively.