1. The H37 strain of human tubercle bacillus has been dissociated into two variants,—the R, or avirulent, and the S, or virulent, variants.

2. When their dissociation is complete, colony morphology for each is distinctive and typical.

3. The morphology of the individual bacilli of the two variants is different and characteristic.

4. The R variant develops best in an acid medium; the S develops best in an alkaline medium.

5. A method of assortment to facilitate the procedure of forced dissociation has been described.

6. The difference in virulence has been proven by observations on the pathogenesis of the variants.

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