The usefulness of two colorimetric methods for the determination of the susceptibility or resistance of Mycobacterium tuberculosis to rifampin, streptomycin, and isoniazid in liquid medium based on the reduction of 2,3-bis(2-methoxy-4-nitro-5-sulfo-phenyl)-2H-tetrazolium-5-carboxanilide (XTT) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was investigated. The agar proportion method was used as the reference method. Results obtained indicate that the sensitivity of the XTT reduction assay for the detection of rifampin resistance was comparable to that observed, and previously described, for the MTT assay. However, the reduction of XTT yields a water-soluble formazan that can be easily quantified without performing additional steps such as addition of lysing buffer and solubilization. Furthermore, the colorimetric assays, based on the reduction of XTT and MTT for the detection of isoniazid and streptomycin resistance in Mycobacterium tuberculosis, were standardized. The inhibition of MTT and XTT reduction after treatment with rifampin, streptomycin, or isoniazid was directly proportional to the reduction in the number of viable bacteria, and a strain of Mycobacterium tuberculosis could be reported as susceptible or resistant to rifampin, streptomycin, or isoniazid after 3, 6, or 8 days, respectively. The XTT and MTT reduction assays are rapid, reliable, and affordable and do not require the use of radioisotopes. Moreover, they can be performed with common laboratory equipment.