The observations summarized above were made because of the striking associations between peripheral blood eosinophilia and various human diseases. Clinicians caring for patients with asthma have known for years that peripheral blood eosinophilia and marked bronchial and sputum eosinophilia are associated with asthma. As a consequence of studies using the eosinophil granule MBP, we are now able to postulate that eosinophils damage bronchial epithelium during the course of asthma. This model proposes that eosinophils are attracted into bronchial epithelium and degranulate, releasing toxic cationic proteins which cause desquamation and damage to bronchial epithelium. This model explains many of the features of bronchial asthma, but it leaves open the question of the mechanism(s) for eosinophil attraction to the bronchial epithelium. The studies of eosinophils in cutaneous diseases have led to the hypothesis that the eosinophil loses its morphologic integrity in the skin either by degranulation and loss of its characteristic granules or by frank death of the cell. In either event, there is a strong association between deposition of eosinophil granule MBP and cutaneous oedema. Finally, a molecule indistinguishable from eosinophil granule MBP is found in the human placenta.