Minimal inhibitory and bactericidal concentrations (MICs and MBCs) of rifampin (RMP), rifabutin (RBT), rifapentine (RPT), CGP-7040, and P-DEA, were determined for 50 M. avium strains in 7H12 liquid medium radiometrically under various pH conditions. Half were isolated from patients with AIDS and the other half from patients without AIDS but with pulmonary disease. The MICs and MBCs were also determined in 7H12 broth for M. tuberculosis strains. The MIC results obtained with M. tuberculosis strains, and the serum peak levels in humans, were used as standards for interpretation of the MICs and MBCs of the rifamycins for M. avium. The bactericidal activity of all rifamycins for M. avium was substantially lower than for M. tuberculosis. The majority of M. avium strains was within the "susceptible" category, e.g., comparable to susceptible M. tuberculosis strains, when tested with CGP-7040 and RPT, and all of them were "moderately susceptible" when tested with P-DEA. On the basis of in vitro bacteriostatic and bactericidal activity, it seems that three agents, RPT, P-DEA, and CGP-7040 have more potential than do RMP and RBT against M. avium disease.