Although the roles of CD8+ T cells and a major preerythrocytic antigen, the circumsporozoite (CS) protein, in contributing protective antimalaria immunity induced by radiation-attenuated sporozoites, have been shown by a number of studies, the extent to which these players contribute to antimalaria immunity is still unknown. To address this question, we have generated C57BL/6 (B6) transgenic (Tg) mice, expressing K(d) molecules under the MHC-I promoter, called MHC-I-K(d)-Tg mice. In this study, we first determined that a single immunizing dose of IrPySpz induced a significant level of antimalaria protective immunity in MHC-I-K(d)-Tg mice but not in B6 mice. Then, by depleting various T-cell subsets in vivo, we determined that CD8+ T cells are the main mediator of the protective immunity induced by IrPySpz. Furthermore, when we immunized (MHC-I-K(d)-Tg × CS-Tg) F1 mice with IrPySpz after crossing MHC-I-K(d)-Tg mice with PyCS-transgenic mice (CS-Tg), which are unable to mount PyCS-specific immunity, we found that IrPySpz immunization failed to induce protective antimalaria immunity in (MHC-I-K(d)-Tg × CS-Tg) F1 mice, thus indicating the absence of PyCS antigen-dependent immunity in these mice. These results indicate that protective antimalaria immunity induced by IrPySpz in MHC-I-K(d)-Tg mice is mediated by CS protein-specific, K(d)-restricted CD8+ T cells.