1. Bacterium lepisepticum, Type D, inoculated in small amounts into meat extract broth, pH 7.4, under aerobic conditions failed to grow. Larger inoculations underwent a period of lag before the logarithmic growth phase began.
2. In this medium, dissociation of Microbe D to the variant G occurred readily.
3. A lowered oxygen tension in this broth resulted in growth of small numbers of Type D with no lag and great inhibition of the type dissociation process.
4. Similar optimum growth conditions were obtained by adding to extract broth, under aerobic conditions, autoclaved or unautoclaved rabbit blood in very small amounts, 0.0003 per cent, just sufficient to give a positive benzidine test.