This report examines the mechanisms involved in the down-regulation of the immune response in acute viral infection and documents the presence of apoptotic lymphocytes in situ in the spleens of mice during the resolution of the immune response to acute lymphocytic choriomeningitis virus infection. Apoptotic cells were detected by an in situ nucleotidyl transferase assay. Both T and B lymphocytes were shown to be dying in vivo, the latter in clusters. A biphasic occurrence of apoptosis during the course of the acute infection was observed, with elevated levels occurring at day 3 after infection and a second more pronounced peak at day 11 after infection, coincident with the decline of the cytotoxic T lymphocyte response and with the decrease in total splenic leukocyte number. Apoptosis in vivo was detected in lpr mice, suggesting that Fas expression is not imperative for lymphocyte apoptosis in the context of an acute viral infection. Apoptosis in situ and the decline of the T lymphocyte response to acute lymphocytic choriomeningitis virus infection was unaffected by the enforced lymphocyte-directed expression of Bcl-2, a protein that blocks growth factor deprivation-induced apoptosis of lymphocytes in vitro. These results argue that the silencing of the T cell response to acute infection may not be a result simply of growth factor deprivation. The susceptibility of activated T cells to apoptotic death, which has previously been associated with virus-induced immune deficiency, may therefore also explain the en masse elimination of the expanded lymphocyte pool subsequent to an acute viral infection.