The in vitro activity of new quinolones was evaluated against Mycoplasma pneumoniae (10 strains) and Mycoplasma hominis (approximately equal to 70 strains) by agar dilution, and against Ureaplasma urealyticum (approximately equal to 115 strains) by broth dilution. The static effect of pefloxacin, ofloxacin, ciprofloxacin, enoxacin was investigated for all the strains. Rosoxacin was included in the tests for U. urealyticum and M. hominis. Pefloxacin, ofloxacin, ciprofloxacin and enoxacin were within the same range of sensitivity for M. pneumoniae; the minimal inhibitory concentrations (MICs) of the 10 strains were 1 mg/l for ciprofloxacin, 2 mg/l for pefloxacin, MICs range was (0.05-1 mg/l) for ofloxacin and (0.5-4 mg/l) for enoxacin. Ciprofloxacin was the most active compound against M. hominis; MICs range and mode MICs were respectively in mg/l: (0.1-1) 0.5 for ciprofloxacin, (0.2-2) 0.5 for ofloxacin, (0.5-2) 1 for pefloxacin, (0.5-8) 2 for enoxacin, (2-16) 2 for rosoxacin. Ofloxacin was the most active compound against U. urealyticum; MICs range and mode MICs were respectively in mg/l: (0.2-2) 1 for ofloxacin, (0.1-8) 2 for rosoxacin, (0.5-8) 4 for pefloxacin, (1-16) 4 for ciprofloxacin, (2-32) 8 for enoxacin. No difference could be observed between tetracycline sensitive or resistant strains.