The importance of neutrophils in killing extracellular, pyogenic bacteria has long been established. However, there is only indirect evidence for a role for neutrophils in resistance against intracellular organisms. In this study, we directly demonstrate the involvement of neutrophils in defense against Listeria monocytogenes in normal C.B-17 immunocompetent and C.B-17 SCID mice. Because of the lack of sterilizing T-cell immunity, SCID mice are unable to completely eliminate listeriae systemically and become chronically infected. Both immunocompetent and SCID mice treated with a specific neutrophil-depleting monoclonal antibody during the early stages of Listeria infection were rendered remarkably sensitive to the organism, with a high level of mortality resulting from enhanced bacterial growth. At a late stage of infection in SCID mice, however, administration of neutrophil-depleting antibody did not affect mortality. In spite of the neutrophil depletion, other parameters of nonspecific immune function were normal. Macrophage infiltration to the site of infection and macrophage expression of major histocompatibility complex class II molecules were unaffected. Moreover, NK cell functions were normal as measured by infiltration to an infection site and gamma interferon production. These data demonstrate an important role for neutrophils in controlling the acute phase of Listeria infection, cooperating with, and yet independent of, macrophages and NK cells.