Between January 1995 and March 1997, 78 Helicobacter pylori strains were isolated from patients with gastritis and gastric ulcer and their drug-susceptibilities to 8 antimicrobial agents and 3 anti-ulcer drugs were determined. Imipenem was the most active agent and its MICs to all the strains tested were lower than 0.013 microgram/ml. Amoxicillin, cefaclor and minocycline were active against H. pylori with MIC90s of 0.05 microgram/ml, 0.78 microgram/ml and 0.39 microgram/ml, respectively, and no resistant strains against these drugs were isolated. However, resistant strains to clarithromycin (isolation frequency: 9%), erythromycin (13%), ofloxacin (8%) and metronidazole (13%) were isolated. Triple, double and single resistant strains to above 4 antimicrobial agents were noted. No quadruple resistant strain was isolated. Frequencies of those resistance patterns were 14.3% (triple), 28.6% (double), and 57.1% (single), respectively. Seven erythromycin-resistant strains were shown to be cross-resistant to clarithromycin but 3 erythromycin-resistant strains were susceptible to clarithromycin. It seems likely that this phenomenon is caused by the fact that clarithromycin is more active to H. pylori than erythromycin. The MIC90 value of lansoprazole was lower than those of omeprazole and famotidine.