The most important eosinophil-derived mediators which may contribute to the pathogenesis of asthma are: major basic protein, eosinophil cationic protein, eosinophil peroxidase, reactive oxygen species, leukotriene C4, and platelet-activating factor. The potency of the eosinophil to release these mediators can be greatly enhanced by eosinophil mobilizing and activating factors such as granulocyte macrophage colony stimulating factor, interleukins 3 and 5, platelet-activating factor, and perhaps leukotriene B4. For the development of more effective anti-asthma drugs it, therefore, seems relevant to unravel and interfere with the steps of eosinophil activation.