In characterizing the immunochemical and biochemical nature of middle ear effusions (MEE) of chronic serous otitis media (SOM), we have previously shown that MEE contain all major immunoglobulin classes, increased levels of several lysosomal enzymes, decreased total complement, but increased levels of C3. This report extends these observations by showing that MEE also contains C3 proactivator (C3PA) that can be activated by those organisms involved in acute otitis media, which confirms a functional alternate complement pathway. Furthermore, we have demonstrated the presence of soluble immune complexes in MEE using the Raji cell test. These data further support an immune complex mediated etiopathogenesis for chronic SOM.