1. The tubercle bacillus grown on a synthetic (Kirchner's) medium produces fluorescent substances in the course of its growth, part of which is alkali-resistant and its chemical nature undetermined, while the other, the greater part, is alkali-labile and behaves like riboflavin. During the lag period of 8 to 10 days of little or no growth only the alkali-resistant fraction was in evidence. With the beginning of growth there was a progressive increase in fluorescent material, the rate of increase being relatively greater in the alkali-labile fraction.

  2. The addition to Kirchner's medium of small amounts of iron as ferrous sulfate, or 2 per cent sucrose, or both, increased the amount of fluorescent material formed and also stimulated the growth of A27.

  3. Experiments with three riboflavin related compounds have shown that isoriboflavin had little or no effect on the growth of the tubercle bacillus or in antagonizing the formation of fluorescent material by the tubercle bacillus. Diaminophenazine (i.e. dinitrophenazine reduced with iron) inhibited growth of the tubercle bacillus and apparently may have also reduced riboflavin production. The third substance tested, 4-amino-ribitylxylidine, inhibited the growth of the tubercle bacillus when added to Kirchner's medium in the high concentration of 50 mg per cent. When added in concentrations of 1 to 20 mg per cent it appears to have been metabolized by the growing tubercle bacillus in part to riboflavin, and this endogenous source of riboflavin seemed to have little effect on growth.

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