The MICs of 21 antimicrobial agents were determined for 97 clinical isolates of Campylobacter pylori. The beta-lactams (penicillin, ampicillin, cefoxitin and cephalexin), macrolides (erythromycin and azithromycin), quinolones (ciprofloxacin and ofloxacin), nitrofurans, gentamicin and tetracycline all had MIC90 values of less than or equal to 0.5 mg/l. Aztreonam, flucloxacillin, amifloxacin and rifampicin had moderate activity. All isolates were resistant to vancomycin, cefsulodin and amphotericin B. Five percent of the strains were inhibited by 8 mg/l of polymyxin. Of the oral agents tested, the nitrofurans and ampicillin are probably the most appropriate antimicrobial agents. Azithromycin and the oral form of cefuroxime are promising alternatives. Cefsulodin, vancomycin and amphotericin B would be suitable constituents of selective media for isolation of Campylobacter pylori.