The protozoan parasite Theileria parva is the causal agent of an acute, usually fatal disease of cattle characterized by widespread parasitism and destruction of cells of the lymphoid system. Immunity against the parasite, can be induced by various infection and treatment regimes. Because of the necessity to use live organisms, these methods of immunization are cumbersome and there is the added problem of parasite strain heterogeneity. Present evidence suggests that immunity may operate at two levels of the infection, namely, the sporozoite and the macroschizont-infected cell. Antibodies with the capacity to neutralize the infectivity of sporozoites have been produced and have been used to identify a sporozoite surface antigen of approximately 68,000 daltons. The feasibility of using such an antigen for immunization is currently being explored. In cattle undergoing immunization against T. parva, cytotoxic T cells are produced which are specific for parasitized target cells and are restricted by class I major histocompatibility antigens. Similar effectors can now be generated and maintained in vitro and work is underway to obtain cloned populations of these cells. It is hoped that such effector cells may be of use, in conjunction with molecular biological techniques, in identification of the target antigens on the surface of parasitized lymphocytes.