The structure of a slow reacting substance (SRS) from mouse mastocytoma cells was recently reported (Murphy, R.C., Hammarström, S. and Samuelsson, B. (1979) Proc. Natl. Acad. Sci. USA, in press). We proposed that SRS is formed from a previously described unstable epoxide intermediate in the formation of dihydroxylated arachidonic acid metabolites in leukocytes. The term leukotriene is introduced for compounds which like SRS are non-cyclized C20 carboxylic acids with one or two oxygen substituents and three conjugated double bonds. © 1979.