Acute pneumococcal otitis media was produced experimentally in 100% of chinchillas whose middle ear cavities were inoculated with Streptococcus pneumoniae. Although intranasal inoculation with S. pneumoniae led to otitis media in 21% of animals, 66% of the colonized chinchillas that had negative pressure in the middle ear developed otitis media. Intranasal inoculation with S. pneumoniae followed by intranasal inoculation with influenza A virus led to otitis media in 73% of chinchillas, while inoculation with infleunza A virus alone produced otitis media in 4% of animals. Measurement of middle ear pressure by tympanometry showed the influenza virus-infected chinchillas developed negative middle ear pressure before the appearance of clinical signs of otitis media. While direct inoculation of the middle ear with pneumococci produced a purulent effusion within days after inoculation, intranasal inoculation with both pneumococci and influenza A virus resulted in the rapid appearance of a serous effusion, which became culture-positive for S. pneumoniae during the second week after inoculation. Spontaneous resolution of middle ear infection occurred within three to eight weeks after inoculation and was associated with an increase in the level of type-specific pneumococcal antibody in serum and middle ear effusion. The histopathology of pneumococcal otitis media included purulent middle ear effusion, epithelial metaplasia, and subepithelial edema and hypercellularity. The initial infiltration of the subepithelial space with polymorphonuclear leukocytes was superceded by infiltration with mononuclear leukocytes in animals that were observed for longer than eight weeks. Among untreated animals observed for long periods, production of granulation tissue, development of submucosal fibrosis, and osteoneogenesis were observed. Systemically administered pneumococcal capsular polysaccharide vaccine was effective for prevention of type-specific otitis media in chinchillas when vaccinated animals were challenged by intranasal inoculation with pneumococci followed by deflation of the middle ear. Animals that seroconverted with at least a twofold increase in the level of antibody in serum after vaccination had an 87% lower incidence of otitis media than did unvaccinated animals. Protection was associated with high levels of antibody in serum before intranasal inoculation, and higher antibody levels were found in sterile middle ear effusions than in effusions that contained pneumococci.