The in vitro activity of 11 antibiotics against 16 Chlamydia trachomatis strains isolated from genital tract and 2 Chlamydia psittaci strains isolated from pulmonary tract. The CMI determination (the lowest dilution of the drug which inhibited the inclusion development when untreated control give 10(3) IFC) was assessed by growth in cycloheximide treated McCoy cells-antibiotics were added after incubation for 2 hours at 37 degrees C. The CMI values for Chlamydia trachomatis and Chlamydia psittaci are similar. The activity of antibiotics by comparison between CMI90 and serial levels. The CMI90 (mg/l) were: doxycycline = 0.2, minocycline = 0.2, roxythromycine = 0.12, érythromycine = 0.5, spiramycine = 4, rifampicine = 0.016, ofloxacine = 4, ciprofloxacine = 4, pefloxacine = 8, lomefloxacine = 8, fleroxacine = 8. For Chlamydia trachomatis species all the strains have the same sensibility, no resistance was detected.