Population genetics followed by purification suggested that “null” mutation in the mitochondrial E2 isozyme of human aldehyde dehydrogenase (EC 1.2.1.3) occurred in the Oriental individuals who are sensitive to alcohol. This report demonstrates that the Oriental E2, thought to be a “null” mutant, is catalytically active and except for maximal velocity and isoelectric point, identical with Caucasian E2 isozyme. The data presented are not inconsistent with mutation but preclude active site of the enzyme as the point at which alteration has occurred; they are, however, inconsistent with “null” mutation.