Cell-free extracts of M. tuberculosis H37Rv grown in vivo are completely devoid of dehydrogenase activity. This was shown to be due to an inhibitory substance confined only to tubercle bacilli grown in vivo, and as yet unidentified. Similar inhibitory activity was also found in extracts of mouse lungs infected with M. tuberculosis. The inhibitor affected lactic dehydro-genase of cell-free extracts of M. phlei, M. tuberculosis BCG and H37Rv, and succinoxidase of lung homogenates of normal mice.