The eosinophil is characterized by specific cytoplasmic granules that contain a series of cationic toxins able to kill many targets, including helminths, protozoa, bacteria, and other cells. In bronchial asthma, considerable evidence exists that the eosinophil releases granule proteins, especially the major basic protein (MBP), which in turn mediate tissue abnormalities. Among eosinophil-activating factors, IL-5 has been associated with helminth infection and hypersensitivity diseases and would appear to be an attractive target for pharmacological intervention.