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Lymphocytic choriomeningitis virus (LCMV)–specific CD8+ T cell responses are considered to be independent of CD28–B7 costimulation. However, the LCMV-specific response has never been evaluated in B7.1/B7.2-/- mice. For this reason, we decided to study the T cell response in B7.1/B7.2-/- mice infected with two different strains of LCMV, one (Traub strain) typically causing low-grade chronic infection, and another (Armstrong clone 53b) displaying very limited capacity for establishing chronic infection. Using Traub virus we found that most B7.1/B7.2-/- mice were unable to rid themselves of the infection. Chronic infection was associated with a perturbed CD8+ T cell epitope hierarchy, as well as with the accumulation of cells expressing markers of terminal differentiation and being unable to respond optimally to Ag restimulation. Examination of matched CD28-/- mice revealed a similar albeit less pronounced pattern of CD8+ T cell dysfunction despite lack of virus persistence. Finally, analysis of B7.1/B7.2-/- mice infected with Armstrong virus revealed a scenario quite similar to that in Traub infected CD28-/- mice; that is, the mice displayed evidence of T cell dysfunction, but no chronic infection. Taken together, these results indicate that B7 costimulation is required for induction and maintenance of LCMV-specific CD8+ T cell memory, irrespective of the LCMV strain used for priming. However, the erosion of CD8+ T cell memory in B7.1/B7.2-/- mice was more pronounced in association with chronic infection. Finally, virus-specific T cell memory was more impaired in the absence of B7 molecules than in the absence of the CD28 receptor, supporting earlier data suggesting the existence of additional stimulatory receptors for B7., Lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cell responses are considered to be independent of CD28-B7 costimulation. However, the LCMV-specific response has never been evaluated in B7.1/B7.2(-/-) mice. For this reason, we decided to study the T cell response in B7.1/B7.2(-/-) mice infected with two different strains of LCMV, one (Traub strain) typically causing low-grade chronic infection, and another (Armstrong clone 53b) displaying very limited capacity for establishing chronic infection. Using Traub virus we found that most B7.1/B7.2(-/-) mice were unable to rid themselves of the infection. Chronic infection was associated with a perturbed CD8(+) T cell epitope hierarchy, as well as with the accumulation of cells expressing markers of terminal differentiation and being unable to respond optimally to Ag restimulation. Examination of matched CD28(-/-) mice revealed a similar albeit less pronounced pattern of CD8(+) T cell dysfunction despite lack of virus persistence. Finally, analysis of B7.1/B7.2(-/-) mice infected with Armstrong virus revealed a scenario quite similar to that in Traub infected CD28(-/-) mice; that is, the mice displayed evidence of T cell dysfunction, but no chronic infection. Taken together, these results indicate that B7 costimulation is required for induction and maintenance of LCMV-specific CD8(+) T cell memory, irrespective of the LCMV strain used for priming. However, the erosion of CD8(+) T cell memory in B7.1/B7.2(-/-) mice was more pronounced in association with chronic infection. Finally, virus-specific T cell memory was more impaired in the absence of B7 molecules than in the absence of the CD28 receptor, supporting earlier data suggesting the existence of additional stimulatory receptors for B7.