Previous work has shown that agonistic Abs to CD40 (anti-CD40) can boost weak CD8 T cell responses as well as substitute for CD4 T cell function during chronic gammaherpes virus infection. Agonistic anti-CD40 treatment has, therefore, been suggested as a potential therapeutic strategy in immunocompromised patients. In this study, we investigated whether agonistic anti-CD40 could substitute for CD4 T cell help in generating a sustained CD8 T cell response and prevent viral recrudescence following infection with lymphocytic choriomeningitis virus (LCMV). Contrary to expectations, we found that anti-CD40 treatment of MHC class II-deficient mice infected with a moderate dose of LCMV resulted in severe suppression of the antiviral CD8 T cell response and uncontrolled virus spread, rather than improved CD8 T cell immune surveillance. In Ab-treated wild-type mice, the antiviral CD8 T cell response also collapsed prematurely, and virus clearance was delayed. Additional analysis revealed that, following anti-CD40 treatment, the virus-specific CD8 T cells initially proliferated normally, but an increased cell loss compared with that in untreated mice was observed. The anti-CD40-induced abortion of virus-specific CD8 T cells during LCMV infection was IL-12 independent, but depended partly on Fas expression. Notably, similar anti-CD40 treatment of vesicular stomatitis virus-infected mice resulted in an improved antiviral CD8 T cell response, demonstrating that the effect of anti-CD40 treatment varies with the virus infection studied. For this reason, we recommend further evaluation of the safety of this regimen before being applied to human patients.