Infant rats were passively immunized to determine the protective capacity of pneumococcal anticapsular antibodies. Animal-passaged strains of Streptococcus pneumoniae serotypes 1, 4, 5, 6b, 7f, 9v, 14, 18c, 19f, and 23f were used as challenge inocula (1-1500 cfu) in a model of pulmonary infection that resulted in bacteremia, meningitis, and death. From untreated control animals, histologic sections of lung demonstrated infiltrative pneumonia and lung homogenate cultures grew S. pneumoniae at concentrations of 10(3)-10(8) cfu per gram of lung tissue. A type-specific anti-capsular antibody serum concentration of 0.1-1.15 microg/mL resulted in a statistically significant reduction in mortality compared with the reduction in untreated controls, except for serotype 14, which required 2.32 microg/mL for a significant reduction in mortality. The serum antibody level that provided 50% reduction in mortality ranged from 0.1-3.5 microg/mL for all serotypes.