The in vitro susceptibility of recent Danish human clinical isolates of Campylobacter pyloridis to cimetidine, sucralfate, bismuth subsalicylate and sixteen antimicrobial agents was determined by an agar-dilution technique. Benzylpenicillin was the most active drug (MIC90 = 0.1 microgram/ml); ampicillin, erythromycin, gentamicin and ciprofloxacin were slightly less active. All strains were resistant to 100 micrograms sulfamethizole, and nalidixic acid also had little activity on weight basis. Of the three anti-peptic ulcer drugs, bismuth subsalicylate was most active (MIC90 25 micrograms/ml), but sucralfate and cimetidine also had antibacterial activity, although only little (MIC90 3200 micrograms/ml).