Minimal inhibitory and bactericidal concentrations (MIC and MBC) of isoniazid and ethionamide were determined in 7H12 broth in experiments with 68 Mycobacterium avium strains and 14 wild drug-susceptible M. tuberculosis strains. MICs of isoniazid for M. tuberculosis were from 0.025 to 0.05 microgram/ml, and for M. avium from 0.6 to greater than 10.0 micrograms/ml. MICs of ethionamide for M. tuberculosis were from 0.3 to 1.25 micrograms/ml, and 42.7% of M. avium strains were within the same range. Isoniazid and ethionamide were highly bactericidal against M. tuberculosis, but they had very low bactericidal activity against M. avium.