The leukotrienes are synthesized from essential fatty acids via a 5-lipoxygenase enzyme. Most is known about the four-series leukotrienes derived from arachidonic acid. Leukotriene B4 is a potent chemotactic agent for leukocytes and it induces neutrophil-dependent increased microvascular permeability. Leukotrienes C4, D4 and E4 are bronchoconstrictors; and potent mediators of microvascular tone and permeability. The leukotrienes have been suggested to have a role in many inflammatory conditions in man in the skin (e.g. psoriasis), the lung (e.g. allergic asthma), joints (e.g. rheumatoid arthritis) and in the heart (e.g. myocardial infarction). Drugs which inhibit the generation and the actions of leukotrienes are under development and are being tested clinically as potential anti-inflammatory agents.