THE slow-reacting substance of anaphylaxis was recognized as a discrete biologic entity ^{1 , 2} three to four decades before it was chemically defined as three specific cysteinyl leukotriene products of the 5-lipoxygenase pathway.^{3 4 5 6 7} In the interim, it was hypothesized that this moiety might function as a proinflammatory mediator of bronchoconstriction and vasopermeability in diseases of immediate hypersensitivity.^{8 9 10 11} Since the chemical characterization of the slow-reacting substance of anaphylaxis as several cysteinyl leukotrienes and the identification and biochemical assessment of the enzymes in the 5-lipoxygenase pathway (Fig. 1 and Glossary), the products of this pathway have been implicated in a much broader variety of functions . . .