Human monocytes activated with human recombinant interferon-gamma inhibit the intracellular multiplication of L. pneumophila. Legionella pneumophila is an aerobic gram-negative bacterium and a facultative intracellular pathogen. The organism is ubiquitous in aquatic environments, where it presumably multiplies intracellularly in protozoa, in a number of which it has been found to multiply in vitro. To proliferate and produce macrophage-activating factors at the appropriate time and place, or to lyse infected macrophages, specifically sensitized lymphocytes must have some means of recognizing the infected cells. Ferritin is the major iron-storage protein in the cell and provides a readily mobilizable store of intracellular iron in mononuclear phagocytes. In vivo, specifically sensitized lymphocytes presumably proliferate and produce macrophage-activating lymphokines or lyse infected macrophages upon recognition of L. pneumophila antigens displayed on the surface of infected mononuclear phagocytes. The activated macrophage, a major effector arm of cell-mediated immunity, strongly inhibits the intracellular multiplication of L. pneumophila.