This study was designed to clarify the role of immune reaction in otitis media with effusion (OME). Immune-mediated OME was induced in chinchillas, and characteristics of the disease were evaluated biochemically, cytologically, and histologically. OME was induced by a single intratympanic inoculation of keyhole limpet hemocyanin (KLH) in animals which had a high serum anti-KLH IgG titer. In addition, OME was maintained for 3 weeks in seven of 17 chinchillas, boosted by intradermal and intratympanic injections at 1-week intervals. Cells in the middle ear effusion (MEE) consisted substantially of neutrophils, a few macrophages, and other cells. The mean histamine and prostaglandin E2 levels in the MEE significantly exceeded the levels in corresponding sera. The lining membrane of the bullae was edematous and thickened due to dilatation and rupture of blood capillaries and cell infiltration. Next, the animals were inoculated with immune complex into the tympanic cavity. MEE appeared in all treated ears within three days after injection. Biochemical, cytologic, and histologic findings of the immune complex-induced OME resembled those of immune-mediated OME. An immune peroxidase method demonstrated that neutrophils in MEE phagocytose immune complexes. Our findings suggest that a perpetual cycle of inflammatory substances may maintain an inflammatory condition in the middle ear.