2. Growth of a virulent pneumococcus upon plain broth of an H ion concentration changing gradually either to the acid or the alkaline side of neutrality was accompanied by an initial rise in virulence with a subsequent fall, the change being more rapid in the alkaline than in the acid medium. The organism under these conditions underwent an alteration in its behavior toward agglutinating sera; although still specific, its agglutinability became much greater than that of the original organism. This change was more pronounced in the alkaline than in the acid medium.

3. The virulence of a relatively virulent culture of a pneumococcus grown on plain broth underwent different alterations when the organism was transferred at intervals of 1, 2, 4, or 8 hours. With the 1 hour interval, there occurred seemingly an immediate decrease in virulence, while at intervals of 2, 4, and 8 hours, there occurred first a rise and then a fall, the rise being greatest with the 8 hour interval of transfer and least with the 2 hour.

4. Meat infusion adjusted to various H ion concentrations—pH = 7, 7.3, and 7.7—and made with different amounts of meat furnished conditions which caused a decrease in virulence, the unfavorable action varying in inverse proportion to the concentration of the infusion. Dextrose added to both plain broth and to meat infusion neutralized to a marked degree the unfavorable action of these media.

5. The growth of pneumococci on the different ingredients of plain broth, with plain broth made from the same supply as a control, revealed the fact that peptone in 2 per cent solution maintained and even increased the virulence of the strain of pneumococcus studied, while meat infusion caused the usual decrease. The rate of decrease with plain broth was slower than with meat infusion, showing that the 2 per cent peptone neutralized to some extent the unfavorable action of the infusion.