AbstractCD28 deficient (CD28−/−) mice were used to study the role of costimulation in the T cell-mediated, IFN-γ-dependent mechanism of resistance to Toxoplasma gondii. These mice were resistant to infection with the ME49 strain of T. gondii. Analysis of the immune response of acutely infected CD28−/− mice revealed that IL-12 was required for T cell production of IFN-γ and this was independent of the CD40/CD40 ligand interaction. A similar mechanism of IL-12-dependent, CD28/B7 independent production of IFN-γ by T cells was also observed in wild-type mice. Interestingly, although chronically infected wild-type mice were resistant to rechallenge with the virulent RH strain of T. gondii, chronically infected CD28−/− mice were susceptible to rechallenge with the RH strain. This deficiency in the protective memory response by CD28−/− mice correlated with a lack of IL-2 and IFN-γ in recall responses and reduced numbers of CD4+ T cells expressing a memory phenotype. Together, our findings demonstrate that CD28 is not required for the development of a protective T cell response to T. gondii, but CD28 is required for an optimal secondary immune response.