Although the observations in this paper on a relatively small number of rabbits represent an investigation which is, as yet, incomplete, sufficiently definite results have been obtained to warrant presentation. Six rabbits were injected with a “rough” and “smooth” form of Duval's chromogenic acid-fast bacillus from the human leprous nodule, having received preliminary inoculations with a filtrate from the cultures of B. lepra used in the experiment. The results of the present investigation offer support for the view that the “S” or “smooth” form produces lesions in the rabbit that in many respects resemble the human leprous lesion, as shown by finding in some of the internal organs, foci of large, vacuolated mononuclear cells, or the so-called lepra cells, epitheloid and an occasional giant cell in a matrix of newly formed connective tissue. Furthermore, the experimental lesion in the subcutaneous tissue and the lesions of the internal organs revealed acid-fast bacilli free in the interstitial tissue and intracellularly in some of the large mononuclear cells. The generalized thickening of the blood vessels, marked loss in weight of the rabbits with the finding of microscopic lepromata gives a picture closely simulating that found in leprosy in man.
Repeated injections with the “R” form by the three routes failed to produce any leprotic lesions in spite of the massive doses used. The evidence points to the “S” or “smooth” form of chromogenic acid-fast bacillus as being the more pathogenic for the rabbit. It would seem that the “R” form is the more saphrophytic; at least it can be assumed that artificial cultivation leads to certain bacilli adapting themselves to an artificial environment. Animal experimentation will show whether the “R” form will not regain its former pathogenic properties.