The object of this study was the evaluation of the activity of miocamycin and other macrolides (erythromycin and josamycin) against 61 Ureaplasma urealyticum, 1 Acholeplasma laidlawii and 21 aerobe mycoplasmas (M. pneumoniae, M. hominis, M. gallisepticum, M. mycoides) and anaerobe mycoplasmas (M. morale and M. salivarium) both clinically isolated and standard reference strains. The minimum inhibitory concentrations (MIC) values for miocamycin ranged between 0.00625 mg/l and 0.4 mg/l (ureaplasmas) and between 0.001 mg/l and 0.0625 mg/l (mycoplasmas, except M. hominis with 0.025 mg/l to 0.25 mg/l). For erythromycin, the MIC values ranged between 0.19 mg/l up to 500 mg/l (ureaplasmas) and between 0.001 mg/l to 0.0625 mg/l (mycoplasmas, except M. hominis with 100 mg/l to 1000 mg/l). MIC values for josamycin ranged between 0.03125 mg/l and 0.5 mg/l (M. pneumoniae only). The sub-MIC treatment (carried out on human pathogenic mycoplasmas only) evidenced growth curve modifications and a decrease of the O2 uptake directly correlated to the drug concentrations.