1. Mice immunized with heat-killed cells of virulent pneumococci (Type I) which have been treated with active preparations of the bacteriolytic enzyme, develop a certain degree of type specific resistance to subsequent infection. This active immunity, however, appears to be due to the small amount of free acetyl polysaccharide present in the suspension of digested bacteria, and is always of a less pronounced degree than that obtained with intact heat-killed cells.
2. Virulent pneumococci killed by heat or iodine when subjected to the action of active preparations of the bacteriolytic enzyme lose the antigenic property of stimulating in rabbits the formation of precipitating antibodies for the type specific polysaccharide.
3. The enzyme prepared from S or R pneumococci, irrespective of type derivation, is equally effective against the capsular polysaccharide antigen of any specific type of this bacterial species.
4. The inactivation of the capsular polysaccharide antigen is observed when the cells are merely rendered Gram-negative, without being caused to undergo actual disintegration or lysis.
5. These observations emphasize the importance of minimizing the chances of alterations due to the action of cellular enzymes in the course of preparation of the cell suspension to be used as immunizing agents.