This paper briefly summarizes some morphological, biochemical, and physiological features of the pneumococcal cell wall and plasma membrane that may have a bearing on the interactions between a pneumococcal pathogen and components of an invaded host. Although the most extensively studied aspects of these interactions involve the capsular polysaccharides, recent evidence indicates that the "deeper" layers of the cell envelope also can participate in important events of pathogenesis. Emphasis is placed on information that may be useful to colleagues interested in further probing and identifying components of the pneumococcal cell surface that take part in the complex process of pathogenesis of pneumococcal disease.