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To examine the effect of first-line and second-line anti-tuberculosis agents on the ability of fluoroquinolones to kill mycobacteria.A clinical isolate of Mycobacterium tuberculosis and a laboratory strain of Mycobacterium smegmatis were grown in liquid medium and treated with a fluoroquinolone in the presence or absence of anti-tuberculosis agents. Bacterial survival was determined by viable colony counts on agar medium.When moxifloxacin activity was examined in two-drug combinations containing traditional anti-tuberculosis agents, activity was greater than either compound alone with isoniazid, capreomycin and low, but not high, concentrations of rifampicin. Cycloserine contributed no additional activity, and ethambutol interfered with the lethal action of moxifloxacin and gatifloxacin. Experiments with M. smegmatis confirmed that both rifampicin and ethambutol reduce fluoroquinolone lethality. Moreover, ethambutol increased the recovery of fluoroquinolone-resistant mutants newly created by ethyl methanesulphonate treatment.The intrinsic bactericidal activity of C-8-methoxy fluoroquinolones can be adversely affected by some agents currently used for treatment of tuberculosis.