After implantation of approximately 10(3) Listeria monocytogenes organisms into the lungs, mice develop an acute pneumonitis with dissemination of infection to a mediastinal lymph node (MedLN), liver, and spleen. The infections in a MedLN and spleen resolve in approximately 7 days, but the lung infection persists for a few days longer. Pneumonitis is accompanied by a lymphoproliferative response in a MedLN and spleen, and immunity to Listeria is conferred adoptively with MedLN and spleen cells but not with mesenteric lymph node cells. Although the spleen appears to be the major repository of sensitized lymphocytes, splenectomized mice combat Listeria pneumonitis as effectively as normal mice. It is concluded that the induction of immunity to lung infection with L. monocytogenes is efficient and that the cause for the rather protracted pneumonitis is due to a defect in the expression of the cell-mediated immunity effector mechanism.