In this preliminary study we report the activity of 3-methyl-9-substituted-6-oxo-6,9-dihydro-3H-[1,2,3]-triazolo [4,5-h]quinolone-earboxylic acids and their esters as a new class of antiinfective agents against MDR Mycobacterium tuberculosis. In antitubercular screening against H37Rv and 11 clinically isolated strains of M. tuberculosis several derivatives (1o,3a,c,i,j,p) showed MIC90 in the range 0.5-3.2 mu g/mL. 3c showed no cytotoxicity and proved to be the most potent derivative exhibiting MIC90 = 0.5 mu g/mL against all M. tuberculosis strains and infected human macrophages (J774-A1) tested. (c) 2007 Elsevier Ltd. All rights reserved.