Various new quinolones were measured for their in vitro antimicrobial activitiesagainst Mycobacterium avium and M. intracellulare by the two-fold dilution methodsusing two types of media, 7H11 agar medium and 1% Ogawa egg medium. The MIC90s oftest quinolones determined on 7H11 agar medium and Ogawa egg medium were as follows.M. avium (20 strains): ofloxacin (OFLX), 50 and 50μg/ml; ciprofloxacin (CPFX), 12.5and 25 μg/ml; sparfloxacin (SPFX), 6.25 and 12.5, μg/ml; fleroxacin (FLRX), 50 and 50μg/m1; Y-26611, 100 and 100μg/ml; OPC-17116, 12.5 and 50μg/ml, for 7H11 agar and Ogawa egg medium, respectively. M. intracellulare (20 strains): OFLX, 50 and 50μg/m/;CPFX, 25 and 25μg/ml; SPFX, 12.5 and 12.5, μg/ml; FLRX, 50 and 50μg/ml; Y-26611, >100 and>100μg/m/; OPC-17116, 12.5 and >100μg/ml, for 7H11 agar and Ogawa eggmedium, respectively. Thus, the MIC values determined by the 7H11 agar medium werecomparable to those by the 1% Ogawa egg medium, except for OPC-17116. Moreover, fromthe above MIC values, it can be regarded that the potencies of in vitro antimicrobialactivity of test quinolones against M. avium are in the order of SPFX>CPFX OPC-17116>OFLX>FLRX>Y-26611 and those against M. intracellulare are in the order of SPFX>OPC-17116≅CXPFX>OFLX=FLRX>Y-26611. There was found the tendency that M. aviumhas somewhat higher susceptibility to these quinolones as compared to M. intracellulare.