Various aspects of the host's response to infection by Streptococcus pneumoniae are reviewed. First, the structure of the bacterium is described, with a focus on those elements that are related to its immunogenicity and pathogenicity. The epidemiology of the pneumococcal serotypes, which are differentiated by the molecular structure of the capsule, is considered briefly, and several key points are emphasized--for example, that some pneumococcal types are more pathogenic than others; that variations in biological behavior are based on differences in the chemical composition of the capsule, although the particular factors determining virulence are not known; and that cell wall peptidoglycan, a structure common to all pneumococci, plays an important role in the inflammatory reaction in the tissues. Next, current views on the pathogenesis of pneumococcal infections are discussed. Finally, the roles of type-specific antibodies to the capsular polysaccharide and of complement in the opsonization of pneumococci and the clearance of these bacteria from the bloodstream by the spleen and liver are reviewed.