Liposome-encapsulated amikacin shows significantly greater inhibitory activity against the survival of Mycobacterium avium complex inside mouse peritoneal macrophages than the free drug. Similar results were obtained whether the drug was added simultaneously with, 48 h prior to, or 48 h after the addition of mycobacteria to the macrophages. These observations support the hypothesis that the in vivo intravenous delivery of liposome-encapsulated amikacin results in the localization of the antibiotic in phagosomes containing mycobacteria inside resident macrophages of the liver and spleen.