Qualitative methods of susceptibility testing have been employed widely around the world during the last 37 years and have proved to be a generally reliable tool to guide the chemotherapy of tuberculosis with conventional antituberculosis drugs. However, we believe there is a need now for application of modern quantitative methods to mycobacteriology: (1) for susceptibility testing of Mycobacterium avium complex, (2) for evaluation of the bactericidal activity of the antimycobacterial drugs, (3) for evaluation of the inhibitory and bactericidal activity of drug combinations, (4) for search and evaluation of new drugs against both Mycobacterium tuberculosis and Mycobacterium avium complex. Newly developed technology (radiometric automated bacterial growth recording in the BACTEC system) has opened new opportunities for development of quantitative methods in mycobacteriology using liquid culture medium. This paper analyzes the potential applications to mycobacteriology of 4 quantitative criteria: Minimal Inhibitory Concentration (MIC), Minimal Bactericidal Concentration (MBC), Fractional Inhibitory Concentration (FIC), and Fractional Bactericidal Concentration (FBC).