Findings of recent immunologic studies on otitis media with effusion indicate that antibodies in middle ear effusions can either originate from serum and/or from local production in the middle ear cavity and Eustachian tube. Determination of specific antibody activity of different immunoglobulin classes in effusions and sera against certain bacterial antigens may aid in a better understanding of the pathogenesis of otitis media with effusion. A radioimmunosorbent assay was employed in the present study to determine specific antibody activity against streptolysin or staphylolysin. Although these antibody activities were mainly limited to IgG and IgA class antibodies in effusion as well as in serum, it was also found that SIgA of various types of the effusion possesses the antibody activity against these exotoxins. Findings of this study suggest that a local immunity functions in the middle ear cavity of patients with otitis media with effusions and that bacterial infection may contribute to the development of middle ear effusion in certain cases.