We have recently shown that 34 different Helicobacter pylori strains of human and three of animal origin contain alcohol dehydrogenase (ADH). Isoelectric focusing of the enzyme showed activity bands with pI at 7.1-7.3, a pattern different from that of gastric mucosal ADHs. The Km value of H. pylori ADH for ethanol oxidation ranges from 64 to 104 mM. Although H. pylori ADH was capable of utilizing both NADP and NAD as cofactors in alcohol oxidation, it showed a strong preference for NADP over NAD. At neutral pH H. pylori ADH was more effective in aldehyde reduction than in alcohol oxidation. Distinct findings suggest that H. pylori ADH could be a metabolic enzyme taking part in ethanol production by fermentation. It is a rather abundant enzyme comprising approx. 0.5% of all bacterial cytosolic proteins. Therefore, the enzyme presumably has a basic role in the functions and maintenance of H. pylori. 4-methylpyrazole inhibits H. pylori ADH, and suppresses its growth during culture. Bismuth compounds that are commonly used in the treatment of H. pylori associated gastric diseases appeared to be potent inhibitors of H. pylori ADH. Owing to its high specific activity for ethanol (14 U mg-1) under physiological conditions H. pylori ADH can also effectively produce acetaldehyde at moderate ethanol levels. This reversed function of the enzyme and the production of the toxic and reactive acetaldehyde could account for at least some of the gastrointestinal morbidity associated with H. pylori infection. H. pylori lacks aldehyde dehydrogenase activity and can therefore not remove acetaldehyde at least by this pathway.