We have recently investigated the specificity of T cells induced in a human volunteer and a chimpanzee immunized by multiple exposures to the bites of large numbers of malaria-infected mosquitoes. T cell lines and clones have been obtained from a human volunteer immunized with Plasmodium falciparum sporozoites. These CD4+ T cell clones specifically recognize the native circumsporozoite (CS) protein expressed on sporozoites, as well as bacteria- and yeast-derived recombinant falciparum CS proteins. The epitope recognized by the sporozoite specific human T cells mapped to the 5' repeat region of the CS protein and was contained in the NANPNVDPNANP sequence. A T cell line has also been isolated from PBL obtained from a chimpanzee immunized by multiple exposures to the bits of P. vivax infected mosquitoes. The CS-specific chimpanzee T cells were used to identify a T cell epitope within a repeat region of the P. vivax CS protein.