A rat model was used to investigate immunological events in the middle ear mucosa during pneumococcal acute otitis media (AOM). Twelve healthy male Sprague-Dawley rats were challenged in the right middle ear with Streptococcus pneumoniae type 3, the presence of AOM being confirmed by otomicroscopy. Randomly selected rats, four at a time, were sacrificed on days 7, 14 and 56 after bacterial challenge. Immunohistochemical staining for IgG, secretory IgA (SIgA), and IgM was performed on tissue specimens from four separate locations in the middle ear and tubal mucosa. Immunoglobulins, especially IgG, were found around blood vessels in the middle ear mucosa. Immunoreactive lymphoid cells of all three Ig classes investigated, undetectable before bacterial challenge, appeared within 7-14 days after middle ear challenge, and the location of these cells in the middle ear mucosa suggests the presence of IgG, SIgA, and IgM, respectively. On the other hand, reactivity with anti-SIgA (but not with anti-IgG or anti-IgM) in Eustachian epithelial cells, and also in the subepithelial glands of tubal mucosa was present both before and after challenge. The results suggest that the middle ear mucosa is an immunoreactive site only after it has been activated with pathogens. In contrast, the tubal mucosa exhibits immunological activity also prior to the antigenic stimulation of present interest.