The in vitro activity of ciprofloxacin against 5 strains of Chlamydia trachomatis and 5 strains of Ureaplasma urealyticum was tested. Both C. trachomatis and U. urealyticum showed a certain degree of variability in their susceptibility to the drug. The minimum inhibitory concentration (MIC) of ciprofloxacin against C. trachomatis was 1 micrograms/ml, whereas the minimum bactericidal concentration was greater than or equal to 10 micrograms/ml. The MIC of ciprofloxacin against U. urealyticum was 1 micrograms/ml for 3 strains and higher than 10 micrograms/ml for two strains.