1. A simple technique is described for studying the oxidase action of bacteria by means of the oxidation of p-aminoleucomalachite green.

2. It is shown that pneumococci under aerobic conditions produced an oxidase when grown on suitable medium. The sera of any of seven different animal species constitute such a medium, the degree of oxidation by the pneumococcus depending upon the animal from which the serum was taken—rat, guinea pig, rabbit, horse, man, cat, and chicken in order of diminishing suitability.

3. Conditions favoring the oxidation of p-aminoleucomalachite green by a single strain of pneumococci are: the presence of a slight amount of hemoglobin, dextrose, H ion concentration on the add side, and heating of fresh serum for 30 minutes at 56°C. Conditions preventing the oxidation are: sterilized meat infusion, 1 per cent peptone, plain broth, a high concentration of hemoglobin, and absence of oxygen. In a quantitative fashion, meat infusion, 1 per cent peptone, and plain broth interfere with the suitability of serum as a substratum of oxidase production by the pneumococcus.

4. Twenty-three microbic species were studied with reference to oxidative power. They were grown upon 10 per cent horse serum, with and without dextrose, upon 10 per cent guinea pig serum, and upon plain broth. Only three of the twenty-three gave evidence of oxidative power as tested by p-aminoleucomalachite green; namely, the pneumococcus, Streptococcus viridans, and Streptococcus hæmolyticus. Among the strains, of these three pneumococci gave the most intense reaction, after which Streptococcus viridans and Streptococcus hæmolyticus follow in the order named, but with a noticeable variation among the different strains of Streptococcus hæmolyticus.

5. Hemolytic streptococci of human and bovine origin were studied. The only variation in the type of reaction was manifested by the streptococci of milk and cheese origin. Strains from these sources showed definitely the least oxidase activity. Streptococci from mastitis and cow's udder were indistinguishable by the test from the hemolytic streptococci of human origin.

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