T lymphocytes are believed to play a major role in protection against malaria. Previous experiments using in vivo depletion of CD8+ T cells, reconstitution with CD8+ T splenic cells, and adoptive transfer of CD8+ CTL clones demonstrated that protection against the exoerythrocytic stage of the murine strain, Plasmodium berghei malaria, was CD8+ T cell-dependent. Despite evidence for the critical role of CD8+ CTL, neither the cellular nor the molecular requirements for CD8+ T cell induction or for recognition of malaria Ags are known. In this study, we wished to define the role of CD8+ T cells and MHC class I molecules by using the P. berghei malaria attenuated sporozoites (SPZ) protection model in beta 2-microglobulin (beta 2m) knockout (-/-) mice. In contrast to observations that beta 2m-/- mice are resistant to many infectious diseases by compensatory mechanisms involving non-class I-restricted T cells, we found that beta 2m-/- mice failed to be protected against P. berghei SPZ, although immunization with attenuated SPZ induced production of IL-2, INF-gamma, anti-circumsporozoite protein IgG, and proliferative T cells. The lack of compensatory mechanisms involving non-CD8+ T cells was particularly evident in the failure to adoptively transfer protective immunity with wild-type SPZ-immune splenic T cells. From our data it can be concluded that CD8+ T cells induced during immunization with attenuated SPZ must recognize liver-expressed Ags presented by class I molecules to engage effectively in a response leading to destruction of the malaria parasites.