The in vitro activities of 12 macrolide compounds against 28 Mycobacterium avium complex strains isolated from patients with acquired immunodeficiency syndrome were determined by the conventional proportion method and by the BACTEC method. Clarithromycin (A-56268; TE-031), a new macrolide compound, was the most active agent tested, inhibiting 90% of strains at an MIC of 4 micrograms/ml by the BACTEC method. Roxithromycin (RU 28965) and erythromycylamine inhibited 90% of strains at 16 micrograms/ml. The organisms showed high levels of resistance to most other macrolide compounds.