1. The synthesis of ribonucleic acid, desoxyribomicleic acid, and protein in S. muscae has been studied: (a) during the lag phase, (b) during the early log phase, and (c) while the cells are forming an adaptive enzyme for lactose utilization.

2. During the lag phase there may be a 60 per cent increase in ribonucleic acid and protein and a 50 per cent increase in dry weight without a change in cell count, as determined microscopically, or an increase in turbidity.

3. During this period, the increase in protein closely parallels the increase in ribonucleic acid, in contrast to desoxyribonucleic acid, which begins to be synthesized about 45 minutes after the protein and ribonucleic acid have begun to increase.

4. The RNA N/protein N ratio is proportional to the growth rate of all S. muscae strains studied.

5. While the RNA content per cell during the early log phase depends upon the growth rate, the DNA content per cell is fairly constant irrespective of the growth rate of the cell.

6. Resting cells of S. muscae have approximately the same RNA content per cell irrespective of their prospective growth rate.

7. While the cells are adapting to lactose, during which time there is little or no cellular division, there is never an increase of protein without a simultaneous increase in ribonucleic acid, the RNA N/protein N ratio during these intervals being approximately 0.15.

8. Lactose-adapting cells show a loss of ribonucleic acid. The purines-pyrimidines of the ribonucleic acid can be recovered in the cold 5 per cent trichloroacetic acid fraction, but the ribose component is completely lost from the system.

9. The significance of these results is discussed in relation to the importance of ribonucleic acid for protein synthesis.

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