Activated macrophages from mice which were chronically infected with Toxoplasnma gondii or Besnoitia jellisoni, or which had received Freund complete adjuvant, had an enhanced capacity to to kill intracellular Toxoplasma. Enhanced killing by activated macrophages was demonstrated by decreased incorporation of isotopically labeled uridine by intracellular Toxoplasma and by inhibition of plaque formation. The latter resulted from lack of proliferation of the intracellular Toxo-

The ability of poly-L-glutamic acid (PLGA) to induce resistance to infection with an intracellular bacteria was examined. PLGA conferred significant protection in mice against subsequent challenge with an ordinarily lethal inoculum of Listeria monocytogenes. Enhanced resistance, as manifest by a decrease in overall mortality and prolongation of time to death, could be demonstrated for time periods up to 44 days after PLGA treatment. Peritoneal macrophages from PLGA-treated animals were resistant to in vitro challenge with Listeria. Such resistance could be demonstrated for time periods up to 60 days after PLGA treatment. These findings suggest that “activated” macrophages are the effector cells in the PLGA-induced resistance observed.