Mycobacterium tuberculosis var. bovis (BCG) grown on Sauton medium normally forms a pellicle; in the absence of added Zn2+, however, the pellicle sank during incubation and the yield was only about 20% of normal. The Zn2+-starved bacteria were morphologically similar to normal bacteria and were still acid-fast at 7 d as well as 14 d. The Zn2+-starved bacteria had slightly higher free lipid and phospholipid contents than normal; the content of hexoses was lower and proteins slightly lower. The deficient culture medium became opalescent and alkaline. Aspartate and ammonium ions accumulated. There was twice as much protein in deficient as in normal medium; moreover, a class of proteins precipitable at pH 4.5, which was hardly detectable in normal medium, was present in appreciable amounts of deficient medium. The content of aldehydes, measured with yeast alcohol dehydrogenase, was also doubled in deficient medium. Fractionation of acid-soluble aldehydes obtained from deficient medium after acid treatment of a bisulphite precipitate suggested the presence of several complex molecules bearing aldehyde groups. The need for Zn2+ in the medium may be explained by the presence in normal BCG of a Zn2+-requiring NADP-dependent alcohol dehydrogenase activity whose affinity for aldehydes is especially high.