Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract illness in infants. However, the mechanisms leading to resolution of RSV infections are poorly understood. Since alveolar macrophages play an important role in defending the respiratory tract against infectious agents we investigated the interactions of RSV with these cells. Murine alveolar macrophages were challenged in vitro with RSV at different multiplicities of infection. The percentage of macrophages expressing viral antigen was determined by staining with monoclonal anti-RSV antibodies and evaluation by fluorescence microscopy or FACS analysis. The ability of macrophages to support virus replication was measured by a plaque forming assay on HEp-2 cells. Cell lysates of macrophages contained only small amounts of viable RSV in comparison to disrupted HEp-2 cells. The amount of viable RSV as well as the percentage of macrophages expressing viral antigen decreased rapidly over time. Activated macrophages had a reduced virus load in comparison to resting macrophages. RSV infected macrophages released biologically active tumor necrosis factor (TNF) in a virus dose dependent manner. In contrast, a high virus inoculum resulted in reduced microbicidal activity and oxygen radical production. Our results suggest that RSV infection influences different functions of alveolar macrophages in various ways. Since TNF is thought to restrict viral replication in several cell types it may play a role in limiting virus replication.