Cross-protection among pneumococcal serotypes within serogroups was measured in the chinchilla otitis media (OM) model because several serotypes that cause OM in children are closely related biochemically. Chinchillas were given tetravalent vaccine composed of pneumococcal capsular polysaccharides (PS; types 6B, 14, 19F, 23F) conjugated to an outer membrane protein complex, and 89% to 96% developed a ≥2-fold serum IgG rise against vaccine PS. Vaccine efficacy was tested by inoculating middle ear hypotympanic bullae with Streptococcus pneumoniae types 6B, 6A, 19F, or 19A. OM severity in the vaccinated groups challenged with types 6B, 6A, and 19F but not 19A was significantly better than in the respective placebo groups. Culture-positive pneumococcal OM occurred in 38%, 62%, 0, and 78% of vaccinated chinchillas challenged with types 6B, 6A, 19F, and 19A, respectively, but in 88% of type 6B- and 100% of type 6A-, 19A-, and 19F-challenged placebo chinchillas.