The antimicrobial susceptibility of Mycobacterium tuberculosis H37Rv and 17 clinical isolates of the same species was determined using a bioluminescence assay to measure adenosine triphosphate (ATP) produced by bacteria in the broth dilution test performed in Dubos-Tween-albumin broth. The results were compared with those obtained with the standard dilution proportional method performed on Löwenstein-Jensen medium. The drugs tested were streptomycin, rifampin, ethambutol and isoniazid. There was high correlation between bacterial count and ATP concentration in a liquid culture of Mycobacterium tuberculosis H37Rv (r = 0.993); when several cultures of the same strain were assayed after different incubation periods the correlation was somewhat lower (r = 0.846). Addition of tuberculostatic drugs in inhibitory concentrations resulted in a reduction in the ATP concentration as compared to control assays without drugs. Results of susceptibility tests performed by the two methods agreed in 66 out of 68 assays. Differences in the inhibitory concentrations determined by the two methods were due to differences in the methods. Whereas the standard method requires 3 to 4 weeks of incubation, results of the bioluminescence assay were available after five days of incubation. Automation of the assay for routine use is possible.