Agar dilution antimicrobial susceptibility testing of Camphylobacter jejuni showed that erythromycin, clindamycin, nitrofurantoin, and gentamicin were the most active compounds, inhibiting 90% of the isolates at a concentration of 1 microgram/ml or less. The frequency of high-level erythromycin resistance was 1%. Erythromycin-resistant isolates showed cross-resistance to clindamycin. All strains were inhibited by chloramphenicol at less than or equal to 8 micrograms/ml. About 20% of the isolates were resistant to tetracycline at 4 micrograms/ml. All strains were highly resistant to novobiocin, bacitracin, vancomycin, and trimethoprim and resistant to rifampin. The minimal inhibitory concentrations (MICs) of metronidazole ranged from less than or equal to 0.5 to 128 micrograms/ml. The susceptibility of strains to sulfamethoxazole and polymyxin B sulfate was markedly influenced by inoculum size. The MICs of polymyxin B sulfate were significantly higher at 42 than 36 degrees C. All strains were inhibited by nalidixic acid at 32 micrograms/ml. In the penicillin group, ampicillin was the most active compound, inhibiting only about three-quarters of the strains at 8 micrograms/ml. The cephalosporins as a group showed only moderate to poor activity, the most active cephalosporin being cefotaxime, which inhibited about 90% of the strains at 8 micrograms/ml. The use of antibiotics in selective media is discussed.