The in vitro bacteriostatic (MIC) and bactericidal (MBC) activities of ciprofloxacin and seven other quinolone derivatives on Campylobacter jejuni from human origin were determined. Ciprofloxacin, pefloxacin and rosoxacin exhibited the best bacteriostatic and bactericidal activities. For the three compounds the MIC90 was less than or equal to 0.33 microgram/ml while the MBC90 was resp. 0.36, 0.56 and 0.56 microgram/ml. The MBC values were always significantly higher than the MIC values (P less than 0.001). An attempt was made to select strains with an induced resistance against the quinolone derivatives