The bactericidal activities of four injectable antituberculosis drugs, streptomycin, amikacin, kanamycin, and capreomycin, against Mycobacterium avium and M. tuberculosis were tested. All four drugs were highly bactericidal against M. tuberculosis, with low MBC/MIC ratios and MBCs significantly lower than the maximum achievalbe concentrations in serum (Cmax). In contrast, all four drugs had very low bactericidal activities against M. avium: the broth-determined MBCs were significantly higher than the Cmax. On a basis of comparisons with the broth-determined MICs found for susceptible M. tuberculosis strains and with the Cmax, about one-third of 100 M. avium strains tested can be tentatively considered as susceptible to three aminoglycosides (streptomycin, amikacin, and kanamycin) but not to capreomycin. In regard to the MBCs and MICs, the three aminoglycosides tested have about identical potentials as drugs of choice in combination with other drugs for chemotherapy of M. avium disease. The low bactericidal activities of these drugs against M. avium in vitro do not exclude their therapeutic usefulness, because they may produce a synergistic effect in combination with other drugs. Such an option is especially promising for patients whose isolates can be considered susceptible on the basis of the MIC. We found no differences in susceptibility to the four drugs tested for M. avium strains (identified by Gen-Probe) isolated from 50 patients with and 50 patients without acquired immune deficiency syndrome.