Murine mastocytoma cells treated with calcium ionophore A23187 produced a slow-reacting substance (SRS) that caused contraction of guinea pig ileum. The SRS was identified as a cysteine-containing derivative of 5-hydroxy-7,9,11,14-icosatetraenoic acid, specifically a thioether linkage at C-8. This compound, named leukotriene C (LTC-1), is structurally related to previously identified epoxy and dihydroxy metabolites of arachidonic acid in leukocytes. LTC-1 is an intermediate in the formation of leukotriene B and is proposed to be a precursor of leukotriene C. The study used isotope incorporation experiments, specific labeling, and enzymatic derivations to confirm the structure of LTC-1. The results suggest a common biosynthetic pathway for these compounds, involving 5-hydroperoxy-6,8,11,14-icosatetraenoic acid.Murine mastocytoma cells treated with calcium ionophore A23187 produced a slow-reacting substance (SRS) that caused contraction of guinea pig ileum. The SRS was identified as a cysteine-containing derivative of 5-hydroxy-7,9,11,14-icosatetraenoic acid, specifically a thioether linkage at C-8. This compound, named leukotriene C (LTC-1), is structurally related to previously identified epoxy and dihydroxy metabolites of arachidonic acid in leukocytes. LTC-1 is an intermediate in the formation of leukotriene B and is proposed to be a precursor of leukotriene C. The study used isotope incorporation experiments, specific labeling, and enzymatic derivations to confirm the structure of LTC-1. The results suggest a common biosynthetic pathway for these compounds, involving 5-hydroperoxy-6,8,11,14-icosatetraenoic acid.