Forty-one infants, who during their first (initial) acute otitis media (AOM) had culturable S. pneumoniae (Pn) in 57 middle ear fluids (MEFs) which prolonged or recurred during the follow-up, were observed for 7-15 months for the presence of Pn, Pn-antigens (Pn-ags) and initial-type/group Pn-antibodies (Pn-abs) in these MEFs. Initially, Pn-abs were found in only 3 MEFs. During the follow-up a total of 20 ears (35%) were, at various time-intervals, positive for Pn-abs; they belonged to all three major Ig-classes, but were often slight in quantity. Most regularly, and often with distinct positivity, Pn-abs were found to type 3, 9N and 14. Secretory abs were detected in 5 ears. Within one month after initial AOM, Pn-abs appeared in 9 of the 41 ears with MEF (22%); six of them cleared soon after that. MEFs with Pn-abs usually did not show initial-type Pn/Pn-ag, but new Pn/Pn-ag could later be simultaneously present in recurrent MEF. So, pneumococcal AOM may cause a local and/or systemically mediated immune response in the middle ear in infants. The response may protect the ear from homologous, but still be present in apparently non-homologous otitis media.