Although omeprazole is an important component in anti-Helicobacter pylori therapeutic regimes using clarithromycin, amoxycillin and metronidazole, the mechanism by which it enhances antimicrobial action is unknown. One potential explanation for this effect is increased antibiotic chemical stability resulting from gastric pH changes induced by co-administration of omeprazole. The chemical stability of clarithromycin, amoxycillin and metronidazole was investigated in aqueous solutions and in human gastric juice collected before and after a 7-day course of omeprazole. Amoxycillin, clarithromycin and metronidazole were prepared in buffered aqueous solutions of pH 1.0 to 8.0 and in gastric juice of pH 2.0 and 7.0. The gastric juice samples were obtained from fasted H. pylori-negative volunteers before and after they had received a 7-day course of omeprazole. All the samples were incubated at 37 degrees C and analysed at intervals by HPLC. Amoxycillin, clarithromycin and metronidazole were stable in aqueous solutions of pH 4.0-7.0, pH 5.0-8.0 and pH 2.0-7.0, respectively. At pH 2.0, the degradation half-lives were 19.0 +/- 0.2 h, 1.3 +/- 0.05 h and 2200 +/- 1100 h, respectively. In gastric juice samples of pH 2.0, the degradation half-lives were 15.2 +/- 0.3 h, 1.0 +/- 0.04 h and > or = 800 h, respectively. The half-lives of the drugs in the gastric juice samples of pH 7.0 were all > 68 h. The co-administration of omeprazole with amoxycillin or clarithromycin is likely to increase the chemical stability of amoxycillin and clarithromycin in gastric juice. Clarithromycin degrades rapidly at normal gastric pH (1.0-2.0) but amoxycillin and metronidazole are sufficiently stable at this pH to maintain an antibacterial concentration in the stomach.