Infection of mice with lymphocytic choriomeningitis virus (LCMV) causes a major expansion of CD8+ T cells followed by a period of immune downregulation that coincides with the induction of lymphocyte apoptosis in the mouse spleen. CD95 (Fas) and its ligand are important for regulating peripheral T-lymphocyte numbers and can mediate apoptosis of mature T lymphocytes. We infected CD95- and CD95L-deficient mice (lpr and gld, respectively) with LCMV to determine if the immune downregulation that occurred following resolution of the LCMV infection was due to a CD95-dependent apoptotic mechanism. Lymphocytes from LCMV-infected lpr and gld mice were capable of normal T-cell expansion and cytolytic function but were, in contrast to activated cells from normal virus-infected mice, relatively more resistant to T-cell receptor-induced apoptosis in vitro. However, in vivo there were significant numbers of apoptotic cells in the spleens of lpr and gld mice recovering from the infection, and the T-cell number and cytolytic activity decreased to normal postinfection levels. Thus, CD95 is not required for the immune downregulation of the CD8+-T-lymphocyte response following acute LCMV infection.