Murine mastocytoma cells treated with calcium ionophore A23187 produced a slow-reacting substance (SRS) that caused guinea pig ileum to contract. The response was reversed by the SRS antagonist FPL 55712. On the basis of isotope incorporation experiments, spectroscopy, and chemical degradations, the SRS was identified as a cysteine-containing derivative of 5-hydroxy-7,9,11,14-icosatetraenoic acid. This amino acid was attached in thioether linkage at C-6. The SRS is structurally related to previously identified epoxy and dihydroxy metabolites of arachidonic acid in leukocytes. A common feature is the presence of a conjugated triene, and the name "leukotriene" has been introduced to designate 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 also of leukotriene C, which is the SRS identified here.