The important role of mast cells in allergic mediator release and allergy is well established although the possibility that other cell types may also contribute important mediators of their own is not included. Mast cell growth and replication in part under the control of lymphokines providing an important link between cellular immunity and immediate hypersensitivity. Mast cell mediators including histamine, PGE_2 and LTB_4 in turn suppress a number of lymphocyte and phagocytic functions including helper T cell activity suggesting a possible role in controlling immune reactivity. Serotonin may also play a related role by suppressing Ia expression on macrophages. Inhibition of immune reactivity by allergic mediators may be of particular importance in the respiratory tract which is subjected to continual antigenic exposure. A variety of stimuli can release mediators from mast cells including antigen and IgE, complement fragments, Iymphokines and monokines, drugs, hyperosmolar solutions and proteolytic enzymes. In the case of IgE mediated reactions cross-linking of IgE molecules is sufficient to produce release. Sensitization of mast cells and basophils by IgE takes place through glycoprotein receptors. This IgE binding molecule has been purified and attempts are now being made to determine its molecular structure by DNA cloning and sequencing. The receptor also contains a 30000mw component which may be involved in signal transmission into the cell. One of the early events in stimulated cells is accumulation of diacylglycerol, presumably through activation of phospholipase C. Diacylglycerol may contribute to cell activation both by activation of C type kinases and as a source of arachidonic acid. Diacylglycerol may also participate directly in granule lysis, since part of the diacylglycerol produced may be within the granule membrane itself. Leukotriene generation involves the activation of a 5-lipoxygenase leading to LTB_4 and the slow reacting substances, LTC_4, LTD_4 and LTE_4. The mechanism of activation of the enzyme is still not completely clear although Ca^<2+>, protein kinases and proteolytic enzymes may all be involved. The recently acquired information on mediators and mast cell function may provide new approaches to the treatment of allergy.