Rifabutin (ansamycin LM 427) when given alone in doses of 5, 10, or 20 mg/kg showed limited in vivo activity against acute or chronic Mycobacterium intracellulare infections studied in beige C57B1/6, or S/W mice. Prophylactic studies in S/W mice in which chemotherapy was given for 3 wk prior to challenge showed slight improvement. Combination chemotherapy consisting of rifabutin (10 mg/kg) and clofazimine (20 mg/kg) achieved complete sterilization of M. intracellulare infections in spleen and lung when initiated immediately after challenge. Inclusion of ethambutol did not offer additional benefit. If chemotherapy is initiated 3 wk after challenge (established infections), the activity of this double drug combination is less effective.