The occurrence of IgG, IgM and IgA class antibodies against a type-specific capsular polysaccharide of Streptococcus pneumoniae (Pn) and against a whole cell antigen of Haemophilus influenzae (Hi) and Branhamella catarrhalis (Br) was studied using the ELISA method on middle ear effusion (MEE) samples of 85 patients and paired serum samples of 40 patients during the course of acute otitis media (AOM). Although specific antibodies to all of these three bacteria appeared in MEE during the course of an AOM episode, antibodies against the infecting bacteria of that particular AOM episode were more often prominent. The antibodies were also detectable in the MEE without simultaneous presence in the serum. The middle ear infection was prolonged more often if specific antibodies to the infecting bacterium could not be detected in the MEE obtained at the beginning of the AOM attack. The present study indicates that AOM caused by Pn, Hi or Br may induce both a systemic and a local production of specific antibodies against the causative organisms during the course of otitis media. The occurrence of such antibodies in MEE seems to play a major role in the resolution of AOM.