Helicobacter pylori is associated with retarded healing and recurrence of peptic ulcer. Acid inhibitory agents, including proton pump inhibitors (PPIs), accelerate healing of peptic ulceration. Because epithelial cell proliferation is important for ulcer healing, we studied the effects of H. pylori and lansoprazole, a novel PPI, on gastric epithelial cell growth in vitro. Cell viability was significantly decreased when they incubated with 10(8) CFU/ml H. pylori. Nevertheless, all doses tested, from 10(6) to 10(8) CFU/ml H. pylori inhibited cell growth in a dose-dependent fashion. Co-incubation with AG-2000, an acid-converted derivative of lansoprazole, reversed H. pylori-inhibited cell growth. These results indicate that the antiulcer action of lansoprazole may involve a reversible effect on H. pylori-inhibited cell growth in addition to its well-established acid inhibitory action on parietal cells.