Normal resident and BCG-activated alveolar and peritoneal macrophages from female Swiss Webster mice were compared for their ability to ingest and subsequently control the multiplication of Mycobacterium intracellulare in vitro. Resident peritoneal macrophages failed from the moment of ingestion to control the multiplication of engulfed bacilli resulting in host cell lysis, whereas activated peritoneal macrophages and both resident and activated alveolar macrophages constrained bacterial division for at least 7 days before comparable bacterial multiplication led to phagocyte death. The number of bacilli needed to lyse a macrophage was impossible to determine precisely because viable macrophages commonly contained several hundred mycobacteria. Minimal intracellular bacterial generation times were 20 h for each macrophage type. Differences in the rates of bacterial phagocytosis between both macrophage types, either resident or activated, are intrinsic properties of the macrophages and were not induced by the mycobacteria, because the same patterns of particle ingestion were observed after exposure to latex microspheres.