The in vitro activity of azithromycin against 40 strains of Chlamydia trachomatis, Ureaplasma urealyticum and Mycoplasma hominis was investigated in comparison with erythromycin, roxithromycin and minocycline. All C. trachomatis strains were inhibited by azithromycin at a concentration < or = 0.5 microgram/ml. The initial minimum inhibitory concentration (MIC) of the drug for U. urealyticum was 4 microgram/ml, whereas some resistance against the drug was shown by M. hominis. Erythromycin and roxithromycin presented almost comparable activities, whereas minocycline was slightly more active than macrolides against C. trachomatis (MIC < or = 0.25) and more active against M. hominis (initial MIC < or = 1 micrograms/ml). Only 97% of U. urealyticum strains were susceptible to 8 micrograms/ml of minocycline.