East Coast fever, an acute lymphoproliferative disease of cattle, is caused by the apicomplexan parasite Theileria parva. Protective immunity is mediated by CD8(+) cytotoxic T lymphocytes directed against schizont-infected cells. The polymorphic immunodominant molecule, although an antibody-inducing surface molecule of the schizont, has been hypothesized to play a role in protective immunity. In order to evaluate the immunogenicity of PIM for inducing CTL, cattle were immunized with PIM in isolation from other T. parva antigens, forcing the presentation of PIM-derived epitopes on the MHC class I molecules. Although parasite-specific cytotoxicity was induced in both vaccinated animals, their immune response was clearly different. One animal generated MHC-restricted parasite-specific CTL against PIM while the other calf exhibited a strong PIM-specific proliferative response but non-MHC-restricted parasite-specific cytotoxicity. Only calf 1 survived a lethal sporozoite challenge. This DNA immunization technique with an antigen in isolation of CTL-immunodominant antigens might open possibilities for directing CTL responses against predefined antigens, such as strain cross-reacting CTL antigens.