1. Host defenses against Mycobacterium avium complex (MAC) are poorly defined. Peritoneal macrophages from black and beige mice, and cultured human macrophages were infected in vitro with MAC serotype 1 from an AIDS patient, in the presence or absence of normal or convalescent serum. Bacteria:cell ratio was 1:10. Supernatants and macrophage lysates were cultured 6, 24 and 48 h later to determine the uptake and killing by macrophages. Phagocytosis by activated macrophages, obtained from pre-infected and treated mice or stimulated in vitro with endotoxin, was also studied. 2. Neither convalescent serum nor normal serum caused a significant increase in MAC phagocytosis. 3. Unstimulated macrophages from black or beige mice and humans were incapable of killing the intracellular bacteria. Activated macrophages from all sources phagocytized and killed 80 +/- 4% of the initial inoculum after 48 h in culture. 4. These results demonstrate that activated macrophages are required for optimal intracellular killing of serotype 1 MAC.