MICs for 90% (MIC90s) of 75 Bordetella pertussis strains for amoxicillin, erythromycin, rifampin, and sulfamethoxazole-trimethoprim were 1, less than or equal to 0.12, 1, and 4 micrograms/ml, respectively. Susceptibility rates were all greater than or equal to 93%. Only 17% of the strains were susceptible to tetracycline. The MIC90s of ciprofloxacin, enoxacin, norfloxacin, ofloxacin, and roxithromycin were less than or equal to 0.06, 0.5, 0.25, 0.12, and 0.5 micrograms/ml, respectively. For B. parapertussis, the MIC90s were 16-fold higher with amoxicillin and rifampin and 2- to 4-fold higher with the fluoroquinolones and roxithromycin.