1. The ability of alcoholic-precipitated extracts of beef tissue—brain, spleen, heart, and kidney—to stimulate the growth of Staphylococcus aureus, in vitro, and to convert the yellow S form to a white R variant with altered biochemical characteristics conforming to those of an avirulent organism, has been confirmed.

2. The avirulence of the white R variant has been established by tests in vivo on mice.

3. Staphylococcus aureus infections induced subcutaneously, intraperitoneally, and intravenously in mice responded favorably to brain extract following subcutaneous or oral administration. The mortality was 2 per cent in 444 experimental animals and 81 per cent in 448 control animals.

4. The extracts appeared equally efficient when used therapeutically (mortality 2 per cent of 162 experimental animals and 90 per cent in the control series) or prophylactically (mortality 2 per cent of 282 experimental animals and 76 per cent in 286 control mice). Extracts of brain and spleen were more effective than those of either heart or kidney.

5. Studies concerning the mechanism of action of the tissue extracts indicate that they prevented the formation of toxin by Staphylococcus aureus, and had but little effect on toxin actions.

6. Toxicity tests revealed that the brain and spleen extracts were relatively non-toxic, dosages equivalent to 2 per cent of the body weight being well tolerated. Kidney and heart extracts were much more toxic, producing mortality in dosages as low as 0.3 per cent of the body weight.

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