Histamine has long been known as a bronchoconstrictor agonist, but its role in asthma and airway inflammation is not as clear. Histamine release occurs in response to allergic challenge of mast cells. However, it is now known that mast cells are heterogeneous in responsiveness and sensitivity to antiallergic agents. Other cells, such as alveolar macrophages, neutrophils, eosinophils, and lymphocytes, can also be activated during allergic and inflammatory reactions. Damage to the airway epithelium would reduce the release of an epithelium-derived relaxant factor and thus increase airway smooth muscle tone. Released mediators, including eicosanoids, platelet-activating factor (PAF), complement fragments, and neurotransmitters, also contribute to inflammation in the airways and the interaction of all these mediators complicates the interpretation of the role of individual mediators. Not only can individual mediators affect the biological activity of other mediators, but their synthesis and release can be modulated as well.The present symposium was organized to examine the putative role of histamine in airway hyperreactivity and how it might interact with other mediators. Papers were presented on the role of mast cells and epithelial cells in allergic bronchoconstriction. The interaction of mast cells with other inflammatory cells was also reviewed. Further information was presented on the biological characteristics of epithelium-derived relaxant factor. Other papers reviewed the interaction of mediators on bronchoconstrictor activity and in inflammation-induced changes in microvascular permeability. The speakers emphasized that asthma and other forms of airway inflammation are complex disease entities and that a systematic analysis of interactions between the cells and mediators in the airways is necessary to understand the aetiology of these diseases and to develop improved therapeutic regimens.This symposium was supported by the Pharmacological Society of Canada, the Canadian Histamine Association, the Upjohn Company of Canada, and Merck Frosst Canada Inc.