1. The oxygen consumption and catalase activity of both virulent and attenuated strains of tubercle bacilli was reduced significantly by isonicotinic acid hydrazide from 2 different laboratories while the activity of succinic dehydrogenase was unaffected. 2. 1-isonicotinyl-2-isopropylhydrazine slightly inhibited the oxygen consumption of both virulent and attenuated strains of tubercle bacilli, but did not modify the action of succinic dehydrogenase of these cultures. However, the catalase activity of the attenuated BCG strain was significantly reduced, while the catalase activity of the virulent strains was unaffected.