It is well established that eosinophilia both in tissue and in the circulation is characteristic for many clinical conditions. They include diseases associated with immediate and delayed type hypersensitivity. Under these conditions the eosinophil leukocyte is a prominent participant at sites of inflammatory reactions. Through their enzymes they are able to counteract the mediators of inflammation, such as histamine, the slow reacting substance of anaphylaxis (SRS-A) and the platelet aggregating factor (PAF). In addition, eosinophils exert their sepcific functions during parasitic infection, in which they lead to severe damage of parasites. Eosinophils have also been involved in the process of the replenishment of mediators by dampening the synthesis of mediators in mast cells. Accumulation of eosinophils in tissue or in the circulation is due to chemokinesis and chemotaxis. A great number of eosinophilotactic factors can be generated from cells, such as lymphocytes, basophils, mast cells and polymorphnuclear leukocytes. In addition, several serum components have been shown to exert eosinophilotactic properties. The eosinophilotactic factors differ in molecular weight, chemical structure and their specificty. Eosinophilia is therefore a complex phenomenon: It can be modulated by factors which act on the maturation of the eosinophil, on the production and secretion of eosinophilotactic factors and on the responsiveness of the target cells.