Susceptibility of Mycoplasma pneumoniae (10 strains), Mycoplasma hominis (20 strains) and Ureaplasma urealyticum (100 strains) to minocycline and doxycycline was studied in vitro. Minimal inhibitory concentrations were determined using an agar dilution method for M. pneumoniae and M. hominis and a metabolic inhibition test for U. urealyticum. M. pneumoniae strains were highly susceptible to minocycline and doxycycline (MIC less than or equal to 0.1 mg/l). Among M. hominis strains, 17 were susceptible (MIC less than or equal to 0.1 mg/l), whereas 3 were inhibited only by concentrations ranging from 4 to 16 mg/l. Among the 100 U. urealyticum strains, 95 were inhibited by 4 mg/l minocycline and 94 by 4 mg/l doxycycline. Both antibiotics exhibited similar activities against the three species (same ranges, same mode MICs).