Today Mycobacterium avium and Mycobacterium tuberculosis multidrug resistance are responsible for frequent and severe infections in humans and especially in AIDS patients. Irloxacin is a new quinolone derivate, and shows greater activity with an acid pH. It has a good in vitro antimicrobial spectrum against both gram-positive and gram-negative bacteria. We have compared the in vitro activity of irloxacin against mycobacteria (20 M. tuberculosis, 17 M. avium, 5 Mycobacterium bovis, 5 Mycobacterium chelonae, 5 Mycobacterium fortuitum and 1 Mycobacterium gadium)using the Bactec at pH 6.8 and 5.0, with other quinolones (ofloxacin, ciprofloxacin, pefloxacin and 27753 RP). All quinolones tested showed good activity against mycobacteria at pH 6.8 and 5.0. Irloxacin at pH 5.0 had a greater activity agianst M. avium.