Some new quinolones may be used for the treatment of gonococcal urethritis. U. urealyticum is considered as a potential agent of urethritis. This report describes the in vitro antimicrobial activity of seven quinolones against 45 clinical isolates of U. urealyticum. The MIC's geometric mean is (microgram/ml): rosoxacin (1,74), pefloxacin (4,6), oxolinic acid (9), flumequin (12,12), norfloxacin (15,75), nalidixic acid (27). Pipemidic acid is constantly inactive (greater than 128 micrograms/ml). The results of these susceptibility studies provide support for undertaking clinical evaluations of new quinolones against infections with U. urealyticum.