A conferred cross-protection between Haemophilus influenzae type b (Hib) and nontypeable H. influenzae (NTHi) was demonstrated in a previous study of experimental recurrent otitis media. To explore cross-protection further, and to compare oral administration of whole cells with two more conventional routes for vaccination against acute otitis media (AOM), a total number of 79 rats were immunized perorally, subcutaneously and intrabullarly with H. influenzae or pneumococci and thereafter challenged in the middle ear with NTHi or Hib 4 or 9 weeks later. Otomicroscopic changes, bacterial cultures, and serum IgG antibody levels were monitored. The study demonstrated that while peroral administration did not elicit any protection, a resolved pneumococcal AOM could reduce the susceptibility to reinfection with NTHi. In the latter case no cross-reacting antibodies were detected, but the protective rate was 50% or more, and it was comparable with that found after subcutaneous or intrabullar immunization with homologous NTHi or Hib strains. The results suggest that the protection of the rat middle ear mucosa may involve unspecific responses.