This study identifies a slow-reacting substance (SRS) produced by murine mastocytoma cells (MCT) treated with calcium ionophore A23187. The SRS caused contraction of guinea pig ileum and was reversed by the SRS antagonist FPL 55712. Using isotope incorporation, spectroscopy, and chemical degradation, the SRS was identified as a cysteine-containing derivative of 5-hydroxy-7,9,11,14-icosatetraenoic acid, with the amino acid attached via a thioether linkage at C-6. This compound is structurally related to previously identified epoxy and dihydroxymetabolites of arachidonic acid in leukocytes. The name "leukotriene" has been introduced to describe these compounds. Leukotriene A (5,6-epoxy-7,9,11,14-icosatetraenoic acid) is an intermediate in the formation of leukotriene B (5,12-dihydroxy-6,8,10,14-icosatetraenoic acid) and is proposed to be a precursor of leukotriene C, the SRS identified here. The SRS was purified using various chromatographic techniques, including HPLC. Isotope incorporation experiments showed that the SRS was derived from arachidonic acid and cysteine. Chemical degradation and lipoxygenase conversion experiments confirmed the presence of a conjugated triene in the SRS. The UV spectrum of the SRS was similar to that of recently discovered dihydroxy metabolites of arachidonic acid, but with a higher λmax, suggesting an auxochrome allylic to the triene. The amino acid portion of the molecule was attached via a C-S bond, and the SRS was found to have double bonds at ω-6 and ω-9. The structural similarity of the SRS to recently described arachidonic acid metabolites in leukocytes suggests a common biosynthetic pathway involving 5-hydroperoxy-6,8,11,14-icosatetraenoic acid. The name "leukotriene" has been introduced for the conjugated trienes formed, with leukotriene A being the unstable 5,6-epoxy-7,9,11,14-icosatetraenoic acid, leukotriene B being its enzymatic hydrolysis product, and leukotriene C being the SRS described in this paper.This study identifies a slow-reacting substance (SRS) produced by murine mastocytoma cells (MCT) treated with calcium ionophore A23187. The SRS caused contraction of guinea pig ileum and was reversed by the SRS antagonist FPL 55712. Using isotope incorporation, spectroscopy, and chemical degradation, the SRS was identified as a cysteine-containing derivative of 5-hydroxy-7,9,11,14-icosatetraenoic acid, with the amino acid attached via a thioether linkage at C-6. This compound is structurally related to previously identified epoxy and dihydroxymetabolites of arachidonic acid in leukocytes. The name "leukotriene" has been introduced to describe these compounds. Leukotriene A (5,6-epoxy-7,9,11,14-icosatetraenoic acid) is an intermediate in the formation of leukotriene B (5,12-dihydroxy-6,8,10,14-icosatetraenoic acid) and is proposed to be a precursor of leukotriene C, the SRS identified here. The SRS was purified using various chromatographic techniques, including HPLC. Isotope incorporation experiments showed that the SRS was derived from arachidonic acid and cysteine. Chemical degradation and lipoxygenase conversion experiments confirmed the presence of a conjugated triene in the SRS. The UV spectrum of the SRS was similar to that of recently discovered dihydroxy metabolites of arachidonic acid, but with a higher λmax, suggesting an auxochrome allylic to the triene. The amino acid portion of the molecule was attached via a C-S bond, and the SRS was found to have double bonds at ω-6 and ω-9. The structural similarity of the SRS to recently described arachidonic acid metabolites in leukocytes suggests a common biosynthetic pathway involving 5-hydroperoxy-6,8,11,14-icosatetraenoic acid. The name "leukotriene" has been introduced for the conjugated trienes formed, with leukotriene A being the unstable 5,6-epoxy-7,9,11,14-icosatetraenoic acid, leukotriene B being its enzymatic hydrolysis product, and leukotriene C being the SRS described in this paper.