We isolated 165 strains of mycobacteria from the sputum of tuberculous patients. Their biological characteristics and pathogenicity in guinea pigs were tested. The results were as follows.
1. Some mycobacteria were able to grow on the nutrient agar, but the others did not grow on that agar.
2. The biological characteristics of these isolated strains were almost identical with that of M. tuberculosis in respects to niacin test, neutral red test, catalase test and Kf test.
3. Both two kinds of strains, one could grow on nutrient agar and the other could not grow, caused difinite lesions in guinea pigs when these strain were inoculated subcutaneously into the animals. In some lesions caseous necrosis, epithelioid cells and giant cells were found, and it was difficult to distinguish these lesions from the true tuberculous lesions. From the above facts, we believe, that there is no doubt that acid-fast bacilli used in the tests and exhibiting properties not ordinarily associated with M. tuberculosis are nevertheless tubercle bacilli.
4. When the sputum of tuberculous patients, which was treated one or two hours with one per cent NaOH, was immediately planted on the nutrient agar, we found a very small transparent colony by hand lens. This colony had grown a little larger after successive transfers on the nutrient agar and the niacin test of this strain yielded a positive result; and this strain was pathogenic in guinea pigs.