Peptide vaccination induces T cell activation and cytotoxic T cell development. In an effort to understand what factors can improve immune responses to peptide vaccination, the role of 4-1BB (CD137) costimulation was examined, since 4-1BB has been shown to promote T cell responses in other systems. 4-1BBL-deficient (−/−) and wild-type (+/+) mice were immunized with a lipidated lymphocytic choriomeningitis virus (LCMV) peptide NP 396–404. Analysis of peptide-specific responses early after immunization by CTL assay, intracellular IFN-γ staining, and IFN-γ enzyme-linked immunospot assay (ELISPOT) indicated that CD8 T cell responses were reduced 3-to 10-fold in the absence of 4-1BB costimulation. Moreover, when agonistic anti-4-1BB Ab was given, CD8 T cell responses in 4-1BBL−/− mice were augmented to levels similar to those in 4-1BBL+/+ mice. Two months after immunization, 4-1BBL+/+ mice still had epitope-specific cells and were protected against viral challenge, demonstrating that peptide vaccination can induce long-term protection. In fact, 70% of CD8 T cells were specific for the immunizing peptide after viral challenge, demonstrating that strong, epitope-specific CD8 T cell responses are generated after peptide vaccination. In contrast, peptide-immunized 4-1BBL−/− mice had fewer epitope-specific cells and were impaired in their ability to resolve the infection. These results show that immunization with a single LCMV peptide provides long term protection against LCMV infection and point to costimulatory molecules such as 4-1BB as important components for generating protective immunity after vaccination.