1. The bacilli fall into three groups:

Human,

Bovine,

Atypical (comparatively small but quite distinct).

2. The proportion of cases for which the bovine bacillus is responsible varies to some extent with regard to the site of lesion; e.g., they play a larger part in gland than in bone and joint tuberculosis, but the variation due to age differences is much more marked; their responsibility is great during the first five years of life and disappears rapidly after the age of 16.

3. The various strains of bacilli in each group give consistently the reactions characteristic of that group. Strains of bacilli giving intermediate reactions are almost unknown.

4. In no instance have bovine or atypical strains when investigated at intervals in the same case shown change of cultural or virulence reactions toward the human. Further investigation at longer intervals is needed.

5. There is no experimental evidence that the atypical group is composed of bacilli whose characteristics are changing from bovine to human type as a result of human environment.

6. Tubercle bacilli of lowered virulence are found exceedingly rarely except in connection with tuberculous lesions involving the skin; on the other hand in cutaneous lesions bacilli of lowered virulence are common, and this is most marked in lupus, a condition where the more superficial layers of the skin are involved over long periods. This is a further reason for our belief that the skin is an organ particularly valuable in the contest between man and the tubercle bacillus. Scarification and inunction, or intradermal inoculation of tuberculin may prove of exceptional benefit in the treatment of all forms of tuberculosis.