Aerobic Tests. Changes in pH Which Occurred during Incubation.—(a) The pH of sterile specimens of both normal and patients' sera increased, during 24 hours incubation, from 8.0–8.8 up to 9.2–9.6. (b) The pH of patients' sera, inoculated with hemolytic streptococci, progressed in the alkaline direction as did the sterile specimens. (c) The pH of normal sera, inoculated with hemolytic streptococci, pursued a variable course at lower levels than the sterile specimens. The differences in the changes in pH which occurred in streptococcidal sera and in normal controls were dependent upon and secondary to the presence or absence of killing action in the specimens.

Aerobic Tests. Effect of Different Levels of Hydrogen Ion Concentration, Adjusted at Beginning of Experiment, but Uncontrolled during Period of Incubation.—(a) The streptococcidal activity of patients' sera was inactivated when the hydrogen ion concentration of the specimens was adjusted to levels ranging from 6.4 to 7.4. The inactivation of highly potent samples of sera required a greater reduction in pH than did specimens of moderate killing activity. (b) Normal sera, adjusted to pH 9.2 or higher, exhibited a relatively slowly acting bactericidal process, which seemed to represent the toxic effect of alkalinity.

Aerobic Tests. Effect of Glutathione and Sodium Ascorbate.—The streptococcidal action of patients' sera was markedly impaired by the addition of reducing agents. The inactivation was, in general, comparable to that previously obtained under conditions of anaerobiosis. The results, suggesting that some factor pertaining to biological oxidation-reduction may influence the streptococcidal action of sera, have been discussed.

Tests Performed in Closed Jars by Means of Which Relatively Constant pH Was Maintained during the Period of Incubation.—Under the experimental conditions which were employed, the effectiveness of the streptococcidal action of patients' sera was maintained only in alkaline ranges. The most potent specimens of sera were active at lower levels of pH (7.9–8.2) than were other samples (8.4–8.8).

All of the observations reported in this article were made with a strain of hemolytic streptococcus of the beta type, which is highly sensitive to the destructive action of serum from acutely ill patients.

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