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Systemic administration of interleukin (IL)-2 to patients with malignant diseases induces peripheral eosinophilia. In the present study, to clarify the mechanism of eosinophilia induced by IL-2, we examined the changes in the number of eosinophils and eosinophil colony-stimulating factor (Eo-CSF) activity in the pleural fluids of six patients with malignant pleurisy caused by lung cancer or malignant mesothelioma during and after intrapleural administration of IL-2. Results showed that intrapleural administration of IL-2 induced marked eosinophilia in the pleural fluid and moderate eosinophilia in the peripheral blood, and that during IL-2 administration, marked Eo-CSF activity appeared in the pleural fluid before increase in the number of eosinophils, but that this activity did not appear in the peripheral blood. This Eo-CSF activity was inhibited by a combination of anti-IL-5 antibody, anti-IL-3 antibody, and anti-granulocyte/macrophage colony-stimulating factor (anti-GM-CSF) antibody, but not by each antibody alone. Chemotactic activity for eosinophils was also detected in the pleural fluid during IL-2 treatment. These results suggest that eosinophilia in the pleural fluid induced by IL-2 injection into the pleural cavity of patients with malignant pleurisy is due to the Eo-CSF activities of various components, including IL-5, IL-3, and GM-CSF, and chemotactic factors for eosinophils induced locally in the pleural cavity by IL-2.