Labeled CO2 is produced by tubercle bacilli from Cl13-carboxyl- labeled acetic acid and from methyl-labeled acetic acid. C13 is distributed widely in the bodies of the bacilli, their lipids and structural elements, and in the soluble proteins and polysaccharides produced in their growth. Both methyl and carboxyl radicals of acetic acid enter into the major C cycles concerned in the metabolism of the lubercle bacillus. Acetic acid C is concentrated in the lipid to a greater extent than in the protein and carbohydrate elements of the bacilli. Results of experiments with C14 labeled glycerol indicate that glycerol also serves as a source of energy and C for synthesis of all of the major components and products of the tubercle bacillus. Regardless of its position in the glycerol molecule, C14 of glycerol is oxidized readily, appearing in large quantities in the expired CO2. Glycerol C is found also in all of the major fractions of the bacillus. CO2-fixation occurs in the tubercle bacillus. Cl4 from Cl4O2 appears in the soluble protein (tuberculin) and to a lesser extent in the soluble polysaccharide. It is present also in the lipids and body structure of the organism.