Recombinant human interleukin-5 (rhIL-5) was administered repeatedly onto the nasal mucosa of individuals with Japanese cedar pollinosis outside the pollen season. The numbers of eosinophils and epithelial cells and the amount of eosinophil cationic protein (ECP), secretory IgA (S-IgA), and IgA in the nasal lavage fluid increased significantly after the application of rhIL-5. Responsiveness to histamine was also enhanced after the application. When S-IgA was administered onto the nasal mucosa after application of rhIL-5, the amount of ECP in the nasal lavage fluid was significantly more increased. The above findings together with the facts that IL-5 promotes production of IgA, that IgA receptors are present on eosinophils, and that rhIL-5 does not increase release of ECP from eosinophils isolated from the peripheral blood suggest that a series of possible reactions consisting of (1) IL-5--induced production of IgA from the immune-mediating cells, (2) binding of secretory components released from either serous glandular cells or epithelial cells of the nasal mucosa with IgA, (3) release of ECP from eosinophils induced by S-IgA and/or IgA, (4) epithelial damage to the nasal mucosa, and (5) development of nasal hyperreactivity to histamine.