In vitro antimicrobial activity of rifabutin and rifampicin against various mycobacteria, including the Mycobacterium avium complex, was evaluated by the agar dilution method, using 7H10 agar medium. The activity of rifabutin based on MIC50 and MIC90 was higher than that of rifampicin, against all the acid-fast organisms tested. Microbicidal activity of rifabutin against the M. avium complex phagocytosed in mouse peritoneal or alveolar macrophages was greater than that of rifampicin. Both rifabutin and rifampicin had therapeutic effects against murine infections induced by M. avium complex. Rifabutin was somewhat more effective than rifampicin in mice.