Although it has been shown that bile acids possess antibacterial activity against Helicobacter pylori, few reports on their activity have been published. We determined the minimum inhibitory concentration at 72 h of various unconjugated and conjugated bile acids against laboratory standard strains and clinical isolates of H. pylori, and studied morphologic changes of H. pylori under the scanning electron microscope during treatment with deoxycholic acid and ursodeoxycholic acid at the minimum inhibitory concentrations for 24 h. We found that only the unconjugated form of dihydroxy bile acid has antibacterial activity. The minimum inhibitory concentration of deoxycholic acid is 200-400 microg/ml, that of chenodeoxycholic acid is similar to that of deoxycholic acid, and that of ursodeoxycholic acid is 400-800 microg/ml. The morphology of H. pylori changed from its primary rodlike shape to a spherical shape with blebs on the cell surface, and was further degraded to an irregularly condensed mass, following an increase in bile acid. This morphologic change in H. pylori was different from the change to a spherical shape caused by amoxicillin. On the basis of these results, it seems that unconjugated dihydroxy bile acid might be a candidate drug for eradication of H. pylori, but further investigations of the clinical usefulness of bile acid for this purpose should be done.