Recent advances in biochemistry and cell biology have allowed the determination of the structure and biosynthetic pathways of the leukotriene constituents of slow-reacting substance of anaphylaxis. The sulphidopeptide leukotrienes have potent biological actions including effects on smooth muscle, mucus secretion and vascular permeability whereas leukotriene B3 is a powerful chemoattractant for neutrophils. It seems likely that the biological activities of the leukotrienes make a substantial contribution to the pathogenesis of asthma and that research directed towards the development of antagonists or inhibitors of leukotriene synthesis may lead to a major therapeutic advance.