Prostaglandins (PG) and leukotrienes (LT)--arachidonic acid-dependent metabolites--and paf-acether (platelet-activating factor)--an ether phospholipid--are potent mediators of allergic and inflammatory reactions. Their structures, chemical synthesis and biosynthetic pathways have been recently described. These mediators are produced by various cells with proinflammatory activities including the macrophages upon interaction with a specific secretagogue stimulus (phagocytosis of zymosan particles, immune-complexes); in IgE-dependent hypersensitivity reactions; upon interaction with one of these mediators. Formation of these mediators by macrophages depends upon their local environment. Qualitative and/or quantitative variations in their synthesis are observed depending on the tissue they are derived from (alveole or peritoneum) and on the type of inflammation (immunologic specific or not). Their potent biological activities (increase of vascular permeability, smooth muscle contraction, cardiac and vascular effects and/or chemotactism) suggest a role for these mediators in various pathologies.