Solutions of the bodies of pneumococci, obtained by dissolving them in dilute solutions of sodium cholate, by permitting them to undergo autolysis, or by first freezing, drying, and then grinding in salt solution, are actively hemolytic for rabbit, sheep, guinea pig, and human red blood corpuscles. The substance on which this hemolytic property depends is very labile, much of its activity is lost on passing through a filter, and it is destroyed by the action of trypsin. In its properties it corresponds to the substance contained in such solutions which causes the death of guinea pigs on intravenous injection. Its activity is prevented by the presence of minute amounts of cholesterin.
Following the injection of this solution into rabbits and sheep, the sera of these animals acquire increased power of inhibiting its hemolytic action. It therefore possesses antigenic properties.
It may therefore be concluded that the bodies of pneumococci contain a toxin that is hemolytic for red blood corpuscles. This substance is not simply a product of autolysis but undoubtedly exists preformed in the bacterial cell. However, it is not given up to the surrounding fluid as long as the bodies of the bacteria are intact. It may therefore be considered a hemolytic endotoxin.