The leukotrienes are a family of potent mediators of hypersensitivity and inflammation. The application of sensitive and specific radioimmunoassays for leukotrienes has revealed the generation of significant quantities of several leukotrienes by leukocytes exposed to natural stimuli in the absence of exogenous arachidonic acid. The C6 peptide leukotrienes, LTC4 and LTD4, are potent vasoactive and smooth muscle contractile factors. LTB4 and other di-HETEs stimulate polymorphonuclear leukocyte function and suppress T-lymphocyte activities. The effects of LTB4 on T lymphocytes are subset-specific and include the activation of suppressor and cytotoxic T cells. Receptors for LTB4 on polymorphonuclear leukocytes are specific but exhibit considerable heterogeneity of binding affinity and may mediate different functional effects. The complexity of the pathways of generation and metabolism of leukotrienes, and of the leukotriene receptors, suggest that carefully defined systems will be needed to test the effects of pharmacological inhibitors and antagonists.